Darren doesn't yours have a piece of metal connecting the reflector to the latch? My Westinghouse Canada one does and I see two rivets where it would connect, the other end on mine is just held in place with tension on the socket. There is no screw on mine.
@ Niall and Michael: False alarm,not totally Cooperized lol, yep it does have a metal piece connecting to the latch and held by the socket end,I must have been thinking of a different fixture.
I notice this has a lot of similarites with the 1970's OV-25. With the exception of the terminal block, hinge, and other things, but lots of similarites.