Net search found this.
http://www.angelfire.com/rant/allday101/SchwinnCodes1.html
This is the stuff that applies to pre-war Schwinn. Thought Id copy and paste the text...
Donated Pre-War Info.
Parts And Frame Dating By Year.
THIS CHART APPLIES TO CHICAGO SCHWINN`S ONLY.
Thanks to Bob, Joel and Mark
Copyright © 2001
Copy and print this info at will, it`s FREE.
Serial Number Characteristics:
1. Early 1936 and back typically stamped with larger more crude numbers and
letters and are usually from the first third of the alphabet.
2. Early to Mid 1930`s numbers are often UNEVENLY stamped or not in alignment.
Letters are in chronological order berfore the number series.
3. In 1937 the numbers got ALOT tighter together, smallest type setting on
any frame Pre-War or Post-War and very close together and uniform.
Letters in this small type may represent months or bi-monthly production runs
from 1936 ish through 1938. Lowest known letter being B, highest being X
4. Late 1938 or 1939 starts over with possibly a quarterly lettering system
beginning with the letter "A" and in regular Post-War stamping size.
This lettering format runs 1938 ish thru possibly 1942 and from "A" thru "I"
5. Some early postwar 1946 frames stamped with "I" and "J" oops. Then begins the
serial number system reflecting the Post-War frame design changes and starts the
alphabet over at "A", then "B" and so on, up to the letter "D" or "E" in 1948
The rest is known but keep in mind that there are odd letters seemingly tossed
in at certain times with no rhyme or reason to them and Pre-War letters could
have the same type of instances as the known 1948 and up letters do.
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Telltale Frame Designs
1. 1938 The first Cantilever frame.
2. 1939 The first Cycle Truck.
3. 1939 - 1941 Town and Country Tandem 26 X 2 1/8 tires, back seat tube was straight
4. 1941 deluxe had a metal tube welded onto the headtube to protect the tank
from being dented by the springer. 1941 was the ONLY year for this.
5. 1942 No Deluxe Models Made, The Wartime defense model became the standard model.
6. 1954-46 The first Forward facing dropouts, Removable seatpost clamp, Built
In kickstand, Built in chainguard and fender mounts,New Schwinn built front
hub With caged bearings, New truss plate assembly,New front fork (NON braized),
New semi tubular fender braces (not flat sided),Women`s fork yoke now same lenth
7. 1946 to 1951 Town and Country Tandem: Curved rear seat tube. Front hand brake
with rear coaster. OR front hand brake with rear expander. Tire sizes 26 X 1 1/2
OR 26 X 2 1/8 . Single letter T in serial number below crank.
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Telltale Parts
1. Early 1936 marked the end of the large seat post clamps.
2. The Hockey Stick style clamp on chainguard was the most used Guard until 1937,
but was Still used after that until the post-war frame changes that mount it
directly to welded on tabs on the frame rather than Guard mounted clamps.
3. 1936 The first Cycle Lock, was the only one that wasn`t angled to one side.
4. 1937 The first forebrake.
5. 1938 The first Feather style chainguard.
6. 1938 The first Springer.
7. 1939 The first Ribbed chainguard on the Standard and DX models.
8. 1940 The first streamlined teardrop shaped fender light.
9. 1941 The first Non hanging tank.
1941 deluxe had a metal tube welded onto the headtube to protect the tank
from being dented by the springer. 1941 was the ONLY year for this.
10. 1941 The first Full Deluxe Chainguard (like the Phantom).
11. Pre-War Crank usage:
Part no. 501 Crank-mens-chrome plated-28 thread. (dog-leg)
Part no. 502 Crank-ladies and New World-Chrome Plated. (dog-leg)
Part no. 503 Crank-Juvenile-Chrome plated. (dog-leg)
Part no. 7003 Crank New World racer- chrome plated. (Straight, NON dog-leg)