Internal diameter is 9”. The drum and backplate appear to have bolted straight onto the standard hub. There is a hole in the drum to allow access to the adjuster on whatever car this was from - obviously redundant on my Moggie. The casting number on the drum is ADA4092 or 4892.
The bit of Googling I did suggested Wolseley or Riley but then Austin Ambassadors came into the mix. Any ideas? Incidentally the brakes overall are shocking so I still have work to do anyway.
Matt