Sven,
Classicsat has it right, P&S "Surfex" The screw you speak of was missing when I found this guy... I used to have some old Leviton surface mount recepts & switched from the same era... I think my garage swallowed them [Linked Image]

The pushbutton switches are still in pretty good shape as well, for being over 100 years old! The single 4 slot recepts I think were original from when the house was built, they were by far, the most worn out things in that place.. (cords taped to the wall to keep them in the recepts! [Linked Image] ) The duplex 4 slot is one I haven't ever come across before.. Mfg by "Hoosick"?

-Randy