Could that be the serial number of the. Of the frame number or if they've passed it onto Giant (sometimes bike shops. Search Bicycles by Serial Number: Search Bicycles by Serial Number. Please enter the bicycle serial number using only letters and numbers without spaces or other special characters.

Gene from genesbmx.com web site provided this great info on figuring out your old trusty Schwinn;
Schwinn Bike Dating After 1965,
Schwinn serial number sequences, while appearing to be
confusing on first sight, follow several general rules. If the
first letter is a 'A' through 'M' it refers to production month
('A' = January, 'B' = February, etc.).
The letters 'I' and 'O' are not used, to avoid mistaking
them for numbers. If there is only one letter in the first two
positions, the first number or two numbers refer to production
year. Date Codes occupy the first two, three or four positions
in a serial number.
Mass produced Schwinn bikes have been followed
by a production month letter and several numbers, the
first two signifying year of production (81 = 1981, etc.).
Example: D72130 = April 1972. LeTour (and Super LeTour)
numbers (1978-80 and 1982 on) include the letter 'S'
with year of production (8 = 1978, 0 = 1980). Example:
SC94653 = March, 1979. Sting 20' racing bikes have a
production month letter 'T' signifying that it is a tri-oval
frame (with three frame tube ends ovalized). Example:
F1T20404 = June, 1981.
Imported Schwinn serial sequences depend upon country
of origin. Bicycles from Japan have a production month
letter in either the first or second position and a production
year number in the other (first or second) position. Examples:
J677123 = September, 1976. Taiwanese imports have a
separate date code number on the right rear fork end that
includes the letter 'G' followed by a two-digit month code
(01 = January etc.) and two more digits for the production
year (80 = 1980 etc.). Example: G04813 = April 1981.
Also Note that if your bike is after year 2001 the
Schwinn/Questor bankruptcy, It has been found
that ( Pacifc Cycle ) bikes do not use this dating.
Also See: The Schwinn BMX Catalogs Time Line Web Page:
( http://genesbmx.bmxair.com/schwinn/catalogs/bmx.html )
CC - Gene`s BMX Schwinn Page
http://www.genesbmx.com/SchwinnPage.html
Frequently Asked Questions About Schwinn BMX Bicycles.
02 - What Year Is My Schwinn BMX Bike?

Here is an email I got from an actual employee of GT:

• Trek bicycle frame serial numbers (SNs) are stamped into the bottom of the bottom bracket, or (more rarely) at the lower end of the seat tube. The dates coded in the serial numbers, or the dates inferred from the numbers, are dates of brazing or dates on which the new frames were stamped with serial numbers (serialized).
• The bicycle looks good, may have minimal cosmetic (only) blemishes on the frame, fork, components, parts and/or accessories. The bicycle may require minimal service adjustments without the need of replacing any parts. The suspension (if applicable) and braking surfaces are clean with some signs of usage yet free from grooves & pitting.

If the bike frame was manufactured at the Santa Ana CA facility, the first three or four digits of the serial number should be letters abbreviating the model or size of the frame such as 'XL', 'XXL', CR24, or something like that.

The next part of the serial number is eight numeric digits. The first four numbers should be a two digit month followed by a two digit year.

The last four digits are the serialization digits 0000 to 9999 depending on what number the frame was of the years production.

If the frame was made overseas in Taiwan, it gets more complicated. If the number starts with a K, the frame was made by Kinesis.

That's the only one I remember off the top of my head.

There were at least four companies in Taiwan that made the GT lines :

Robinson, Dyno, Powerlite and Auburn included)

The first digit would be a letter designating the factory that made the frame. There might also be a second letter digit specifying the factory that assembled the bike, if applicable. The model and year of production was also coded by a letter, so you won't be able to spot a two digit month and two digit year.
Before I left, I was writing a specification to standardize all of the serial numbers because GT was starting to send most of its US production to other US fabrication shops.
The information might also come in handy for deciphering other makes because the factories in Taiwan make bikes for a lot of different companies.
I am tinkering around with the idea of starting to make my own frames, kind of on a 'one-off' basis. I am also teaching welding at a local community college.
By the way, did I tell you that the guy ran GT's in house fabrication for the last ten years (he recently retired) was none other than Gilbert Axt. Who? Do you remember Race, Inc.? That was Gilbert.
He also did the original PK Ripper for SE Racing. History, man.
It might be a really cool thing if I can track down Gilbert now and interview him for you.
THAT is the kind of stuff that belongs in the BMX Museum!

Giant Bike Serial Number Lookup

Later. I'll write you when I get a chance. Dan Garcia

Giant Bicycle Serial Number Lookup

and here is the updates, thanks to love thegoose! (Michael S)
well we get a good bit of gt serials that could be identified by the owner, if they only had the tips that some of us have learned, so i will post what i know, and anyone with more knowledge can add to it..so here we go.
here is an example to go by. on the dropout you have 'M1' followed by '08922198'..this one is simple.
M1 stands for 'mach one' that is your model gt.
it can also be a 'I' for interceptor and so on.
next on the serial is '0892' this is easy to decode too,
08 means 8th month, which would be august.
then you have '92' next in the lineup, this would be 1992
the last 4 digits, are your build number, this one would be the 2,198th one built.
Here is another example. if your serial is KGCG2297 this one gets a little complicated. but still easy once you learn the code to it.
the first letter is who made the frame itself, in this case it would be 'Kenisis', an overseas company.this could be replaced by another letter but at least we know what that one is there for.
next is the letter 'G'..this isnt always there, but if so, this is where it was assembled, (ever notice the built in usa sticker on your gt or dyno?) this is what that letter is there for. it wasnt made here, but was built here.
next is the letter 'C'..C is the 3rd letter in the alphabet, and march is the 3rd month, so march is the build month.
next is the letter 'G' this is the year. G is the 7th letter in the alphabet,
so this would be a 1987.
here is one more..your serial resembles this:f6030578.
to start, if you have a 'baseball' looking stamp behind your serial, this frame was made overseas...no its not a pacific, so dont cry, or get mad...and lets continue.
on this serial, the first number is the last digit of the year, so 6 stands for 1986
next is '03'....this is the third month..which is march.
so you have a gt built in march of 1986.
but still use the museum as a guide to id the frame, cause sometimes you can find the year, but getting a posotive id on the model isnt possible without knowing which style frame you have.
this doesnt apply to new gt's, but hopefully some people with info on those will chime in. this isnt always going to match your serial, you may have a 'fake'..its not likely but it happens. also on earlier models this can differ. this is for the 2 most popular style serial's i see on here. also, common sense tells you that if you have a suspected 'gt' frame, look it up and compare it in the musuem, knowing which style frame you have will help with the process of id'ing your bike with the serial, especially if the serial has some damage that prevents you from getting the full number.
hopefully this bit of info can be used for everyone, and this will save you from those late nights sitting around waiting for a response to your post looking for an answer to the idendity to your gt...although i do feel smart and proud when i can answer these posts. well, thats it for me, if anyone else has info on serials or if you see a mistake in this guide, tell me so i can edit it. happy holidays people.