The first letter of the model designator reveals the engine series:
G - Servicar three wheeler, 1932 to 1972
E - Overhead valve 61 cubic inch "big twin" (Engine/trans separated)
F - Overhead valve 74 or 80 cubic inch "big twin"
K - Side valve 45 and 55
cubic inch sports bike that replaced the WL in 1953 and was replaced by the sportster in 1957. It had many design features that were carried over to the Sportster.
U - Side valve 74 or 80 cubic inch "big twin"
V - Side valve 74 cubic inch made prior to 1936
W - Side valve 45 cubic inch made 1934 to 1952
X - Sports and special construction. Applied to 1918-1922 opposed twin Sport, 1944 military opposed twin, and
1957 to present Sportster.
There were others, such as the J series (a very nice looking bike that was about the hottest thing around in the 20's) but you're not likely to see them in daily use anymore.
L - high compression. Omitted on low compression early models, and not indicative of compression on current models.
Remaning characters: Model descriptions, eg WG - Wide Glide. Some
A - Military (Army) version (except GA, Servicar without tow bar)
B - Battery start (early models), Belt drive (some later models).
C - Classic, Competition, Custom, various others meanings.
D - Dyna, the newest frame and engine mount design.
E - Electric start
F - Foot shift (when the standard was hand-shift)
H - varied between High performance and Heavy duty. The early FLH for example
produced 5 more HP than the FL and was used primarily for touring and sidecar applications.
LR- Low Rider (though many Low Riders don't include LR in the model ID)
P - Police version
R - Rubber mounted
engine (some models) racing version (other models) e.g. the FXR is a lowrider with rubber mounted engine, WLR is the flat track racer that was very hard to beat in the 30's, 40's, and 50's.
ST- Soft Tail
(without following T) Sports version eg FLHS is an FLHT without the touring package.
T - Touring