Hi Ian,
I'm pretty sure these are P&H lamp brackets. I thought I had two sets, but while one has tapers at the mounting ends - to suit the tapers in the Douglas head stem - the second set are just parallel and 1/2" diameter at the mounting end.
They might be something like BSA?
They're rather nice, but they would be tricky to modify. Check what arrangement you have for mounting on your bike.
Cheers
Leon