Why cab 11, 12, 13?

[David_Hayward (RIP)]

Thanks to Clive! DND papers found!

SPECIFICATION NO. O.A. 85 dated 26-11-41

CAB CODE - CAB TYPE
11 MILITARY PATTERN CAB-ORIGINAL DESIGN
12 MILITARY PATTERN CAB-ALLIGATOR HOOD
13 MILITARY PATTERN CAB-REVERSE SLOPE WINDSHIELD
21 CONVENTIONAL OR MODIFIED CONVENTIONAL CAB
31 CAB-OVER-ENGINE TYPE CAB
41 HALF OR OPEN TYPE CAB

Note:
The second digit of the Code Number which indicates the "history" or "mark" of the cab will be indicated above on the Military Type Cabs. The "history" on Conventional type cabs will refer to the cab as mounted on a specific model vehicle made by a specific manufacturer.

So the # 42 and # 43 cabs were the second and third mark of the half or open cabs respectively.