This seems to be tricky territory...

I have Kodak / Folmer 5x7 sheet-film holders that appear to be post- WW-II, and are black-stained wood, with metal fittings at the dark-slide end.

I also have several 4x5 / 5x7 sheet-film holders that are in nice varnished "furniture-quality" frames.

Then there are the true plate-holders: these are usually varnished, and have the semi-circular notch. ( I am sure there were black-stained plate holders too, but plate photography seems to have declined rapidly by WW-II).

I was playing around with my Rochester Optical cameras and variety of film-holders, and did discover something: the Rochester OPTICAL Co. & Premo cameras all seem to be compatible with modern film -holders (ie: the ridge for the light-trap corresponds to the groove in the camera back). I found one 5x7 Rochester CAMERA Co. (Poco) that has this ridge in a different place, and will NOT engage the light-trap groove in the "standard" view camera back...

At some point in the 1890's, "Rochester Optical Co." and "Rochester Camera Co." combined ( and were eventually bought by Eastman), but I don't know if /when they standardized backs & film-holders between the "Premo" and "Poco" cameras...

I also have a plate-holder that holds a 5x7 plate, but the outside dimensions are too large to fit a standard 5x7 back... ?

I am somewhat dumbfounded by the prices that 5x7 film holders are fetching... quite a bit for a "dead" format (as my friends all tell me)...

Luddite Frank