There were several means of fixing the magneto to the top of a Douglas crankcase, but I don't recognise these fittings. They're sure to be Douglas because the screws - two for each mount - are 15/64-25tpi [Edit - don't know where I got 27tpi from on the original post!].
I guess they're 1920s? The RA used a similar mount but it had only one 17/64-25tpi screw each side and a removable cotter pin, held in place with a split pin, rather than a fixed rivet.
So, what are they off, and how is the magneto clamped down?
Cheers
Leon