I could be wrong, but think the part number for that switch is MT3/LU 351541 Type: RS39
Also listed for that switch is the part No. MT3/LU 351590
The earlier 3 pos. switch had pt. No. LU/359863 Type: MD6L7
Numbers could be mixed up here! more info needed, Lucas stuff is a nightmare to identify, as postwar all numbers were replaced in no logical order, and military numbers not listed at all.
An early G3 Matchless parts list also shows RS39 as the type of switch mounted in the panel of a panel tank G3, but that has a chrome edge! pt. No. is however DF 351-584
There is also the very late war "Test" switch (two models) that number is: MT4LU/31023