Stimsonite 12A was not used on WWII GPW,the Round or Oval bezel Guide with Stimsonite 12 was used.
Stimsonite 12 is proper for WWII, Stimsonite 12A is post war and common to M-Series.
The Difference is in the facets, Stimsonite 12 is coarse, 12A is a finer facet.
For 50's M-Series, the Guide A2 iwith Stimsonite 12A will be found on M-38/M-38a1.
Guide A2 with Guidex lens (similar to Stimsonite 12 A can be found on M-38, M-38A1, M-135/M-211, and Reo M-35 types.
M-37 of the 50's used the round Corcoran Brown with Stimsonire 12A.
PROUD 2011 MVPA PIONEER AWARDEE-HONOR GRAD WHEELED VEHICLE MECHANIC SCHOOL 1960- US ARMY ORDNANCE SCHOOL(MACHINIST) APG 1962 - O-1 BIRD DOG CREWCHIEF - 68 NONSTOP BY THE BOOK ARMY TRUCK-JEEP YEARS LIFETIME AM LEGION DAV 40/8- MVPA 1064 -7 TURNKEY MV'S