[David_Hayward (RIP)]

The cabs, thanks to Clive's information, are:

1 = MILITARY PATTERN CAB
2 = CONVENTIONAL OR MODIFIED CONVENTIONAL CAB
3 = CAB-OVER-ENGINE [FORWARD CONTROL IN UK PARLANCE]
4 = HALF OR OPEN CAB

Then the next digit is the 'mark' or 'history':

11 = MILITARY PATTERN CAB, THUMBSCREW OPENING HOOD BASED ON THE GUY QUAD ANT TO BRITISH W.O. SPEC 36
12 = M.P.C. WITH ALLIGATOR HOOD
13 = M.P.C. WITH REVERSE-SLOPE SCREEN, 30 DEGREES OPENING AT FIRST THEN OCTOBER 1942 [?] 90 DEGREES

41 = IN THEORY OPEN VERSION OF THE 11 CAB..were there any?
42 = OPEN VERSION OF THE 12 CAB WITH MONSANTO SCREENS AS USED IN THE 2-PDR ANTI-TANK PORTEES
43 = OPEN VERSION OF THE 13 CAB AS USED IN THE C60L 6-PDR ANTI-TANK PORTEES
43S = SPECIAL OPEN CAB AS USED IN THE F30B BOFORS FORS MODEL C39QB