AJ wrote:

Anyone know off hand what size thread the screws are for securing
socket
plates to backboxes?

M 3.5 mm

And isn't it Strange how M3, M4, M5 etc are common-as-muck sizes sold at
quid-or-two-gets-you-one-or-two-hundred prices, while the 3.5s having
more or less only this application are Specialist Items selling at 5 or
more times that price? (Well, economics of mass mfrg being what they
are, I s'pose it's not Strange. Presumably M3.5 was the closest in
diameter in metric sizes to the Imperial size previously used (4 BA?)
and it was considered more important to have only the backbox threads
and accessory screws change, leaving the holes in accessory plates
unchanged, than to use a widely-available size...)

Stefek