The model number is in effect the motor number of a bike. The motor number is stamped on the left crankcase. The numerals of the motor number preceding the letters indicate the year. The letters indicate the size and type of the motor.

For example:

41EL2037 would be a 1941 model with a 61 OHV motor.
38U1863 would be a 1938 model with a 74 SV motor.

Below is a list of some of the motor designations:
Model     Years     Description
D     1929 1931     45 SV
DL     1929 1931     High Compression 45 SV
DLD     1929 1931     High Speed 45 SV
E     1936 1950     Medium Compression 61 OHV
EL     1936 1952     High Compression 61 OHV
F     1941 1950     Medium Compression 74 OHV
FL     1941 1965     High Compression 74 OHV
FLE     1955 1965     Special cam police 74 OHV
FLH     1955 1965     High Compression Sport 74 OHV
G     1937 1963     Servi-Car with tow bar
GA     1937 1963     Servi-Car without tow bar
GD     1937 1941     Large body Servi-Car without tow bar
GDT     1937 1941     Large body Servi-Car with tow bar
R     1932 1936     Low Compression 45 SV
RS     1932 1936     45 SV with Side Car
RL     1932 1936     High Compression 45 SV
RLD     1932 1934     High Speed 45 SV
RLDR     1935 1936     High Speed Race 45 SV
U     1937 1948     Medium Compression 74 SV
UA     1939 1940     Low Compression US Army 74 SV
UH     1937 1941     Medium Compression 80 SV
UL     1937 1948     High Compression 74 SV
ULH     1937 1941     High Compression 80 SV
V     1930 1936     Standard Compression 74 SV
VHS     1934 1936     Medium Compression 80 SV
VL     1930 1936     High Compression 74 SV
VLD     1930 1936     High Compression 74 SV
VLH     1934 1936     High Compression 80 SV
W     1937 1952     Low Compression 45 SV
WL     1937 1952     High Compression 45 SV
WLA     1940 1945
1949 1952     US Army Low Compression 45 SV
WLC     1942 1943     Canadian Army Low Compression 45 SV
WLD     1937 1942     High Speed 45 SV

Note that the inch indications above are cubic inches.