G - Servicar three wheeler, 1937 to 1972
E - Overhead valve 61 cubic inch "big twin" (Engine/trans separated)
F - Overhead valve 74,80 or 88 cubic inch "big twin"
FL - 80 to 88 cubic inches and a fat front tire. (Also 74" 1941-82)
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 1930-36. (Also 1935-36 VLH, VHS 80)
W - Side valve 45 cubic inch made 1937 to 1952
X - Sports and special construction. Applied to 1918-1922 opposed twin Sport, 1944 military opposed twin, and 1957 to present Sportster.
The second letter of the model designator reveals the front end (except Sportsters):
X - Narrow tire and sport forks.
L - Wide front tire and Hydra-Glide front forks.
The third letter of the model designator reveals the frame style:
D - "Dyna" frame (with the rubber mounted motor)
HT - "Highway Touring" frame
ST - "Softail" frame
These indicate the model bike within the frame family, or acronyms of the model name.
A - Military (Army) version (except GA, Servicar without tow bar)
B - Battery start (early models), Belt drive (early 80's) Black paint.(1995-6 model, the Bad Boy)
C - Classic, Competition, Custom, various others meanings.
D - Dyna, the newest frame and engine mount design.
DG - Disc Glide
E - Electric start
F - Foot shift (when the standard was hand-shift) and now "Fat Boy®"
H - varied between High performance, hand shift and Heavy duty.
I - Signifies Fuel Injection.
L - Big fat front tire
LR - Low Rider (though many Low Riders don't include LR in the model ID)
N - Nostalgia
P - Police version
R - Rubber mounted engine (some models) racing version (other models)
ST- Soft Tail
S - Springer
S - (without following T) Sports version
T - Touring
WG - Wide Glide
X - Sportster or sportster-type front end and narrow front tire.