To the best of my knowledge, the instrument / panel light switch was the same for the MA, GP, BRC. At least every MA, GP, BRC restoration I have seen has the same switch.
These NOS switches appear on EBay once in awhile. This switch has two threaded holes for two round head screws 8-32 x 3/8 and two lockwashers no.8. However, while you are waiting for a NOS, you can buy a new repro Ford p/n 21C-13740 (Google it and you will find several suppliers selling it for $10 to $25).
The MA switch was p/n 635737. This switch was used on the 1937 – 1940 Willys cars (the 1941-2 Americar used a different switch). Below is a photo of the switch in my MA which I think is correct. The Willys 635737 switch was also used on the CJ2A up to s/n 116421. I think the MA switch was the same as the Ford NOS 21C-13740.
The GP switch was p/n 50-13740. This switch was used on Ford trucks back to 1938.
As I mentioned above, there is a Ford repro switch available p/n 21C-13740 (Google it). You can also find NOS 21C-13740 once in awhile. Here is a previous G503 topic on the GP switch with photosviewtopic.php?f=9&t=178005
The BRC switch was R-10209. I haven’t found any internet discussion on this one. There is a very poor photo in the BRC manual TM-10-1205. All the restorations I have seen use the same switch as in the MA and GP discussed above (NOS 21C-13740).
If anyone has more information please comment.