Hi Douglas.. I do believe the one's that fit in the panel on the tank are the same as the early Austin seven , a friend of mine has fitted one!!
i Have a late 1938 M22 sports and that has a oil clock instead . I don't Know why this is as the 1938/39 m24 has the oil feed tell-tale on the timing side and also they put some in the tank
The top ring of the Austin model has flanges with holes for screws. These can be machined however to look like the ring BSA used. The flanged ring for the Austin model is also screwed on the housing like the BSA type.
Trouble is Douglas they don't come up for sale very often I have been looking for one for at least three years on ebay and when one did they wanted the world and a bit more for it ! this is why i have put a wanted on our lovely helpful forum.. If you do find any one that makes the Give me a shout please regards Gary