They look a lot like an old pair of Sennheisers.
I have a very old pair of HD420SLs, and the back of the earcup looks very much like these, with the three U shaped slots per side, each with two bars inside. An old style open headphone for sure.
Anyway, it looks a lot like my old Sennheiser hd420sls. Its obviously not the same headphone (the strap is a bit different on mine, it doesn't have the two strips going over the top of the strap), but it could be a sennheiser, or a sennheiser clone.
How do they sound? If they really are old sennheisers, then they are probably quite good, the early open headphones they made really popularized open headphones.
peace,
phidauex