Going by stuff brought back to Australia from India during the early 1980s, the .32-40 chambering is fairly common in light DRs of British make. Kynoch loaded the cartridge for several decades. I have a red and yellow box around here somewhere. Also, a friend has a nice Tolley chambered for the cartridge, and I too have seen a couple of WR .32-40s in Darwin.

Note that the proofs on the Tolley read: 32/40 EX Cordite 27 1/2 -- 165 MAX. Twenty grains would seem way too light! You may indeed have a mystery on your hands, Dave!