Robb obviously locally made in Australia. There is a 100 year old company called Bryants in US I think who are into air purification. Maybe they had/have an Australian connection?
Rob, This is the identification plate - nothing flash.
I did some searching when I came across this filter (some years ago) and found very little. The Bryant company was involved in other war effort production - I found reference to them in relation to making parts for Beaufort aircraft and some post-war dairy equipment but zero in relation to these air filters.
The blueprint is dated 4-7-44 so my assumption is that this was a late war local Australian effort probably brought on by a disrupted supply chain to Britain. The mounting hardware is all basic BSW so definitely made to get the job done without anything fancy.
I have seen several of these at swap-meets over the years but had no idea of a potential WM20 connection at the time. I have always wondered if these had different mounting kits for other type of motorcycle given the generic 'Motorcycle Air Filter' on the nameplate but that is pure speculation.