About 30 years ago I bought a couple of tea chests full of Douglas spares. In one were a lot of S1 and other ohv parts. Amongst all this lot was this oil pump. It is adjustable for flow by the small lever on top and has a casting number on it :- RT46.
It measures aprox. 89mm X 45mm on the mounting plate. It's certainly not from an S1. So what's it from ? Maybe somebody out there knows. It seems that most of this stuff had been raced in the past.