katch, Military Mauser Luger Wooden Grips were mostly numbered with the last two digits of the pistol serial number, but there were many that were not numbered. The letters, I believe, were inspection marks and I don't have any idea of their logic. The following is a general guide to grip numbers, but remember, there are exceptions.
K-Date (1934) Early Models had small numbers, Later Models had larger numbers
G-Date (1935) Early Models Numbered, Later Models NOT NUMBERED
1936 S/42 Mostly NOT NUMBERED
1937 S/42 Some Early NOT NUMBERED, Most were numbered
1938 S/42 Numbered
1939 S/42 Numbered
1939 - 42 NOT NUMBERED
1940 - 42 Numbered
41 - 42 Numbered
41 byf Numbered
42 byf Numbered
Hope this helps!