Recently just bought a Bridgeport series I that came with this vice. It doesn't have any normal slots/holes for t-nut bolts and was attached with saw tooth step clamps but is missing the swivel base. It does seem to have a unique feature of an acrylic magnified protractor that I removed and sanded smooth then buffed clear.
Identify brand model of this milling Vice - Album on Imgur
Identify brand model of this milling Vice - Album on Imgur