Suburban 6" Round Magnetic Mag Chuck RMC-6 RMC6 Flat Back Mount. Not a whole lot to say, I've tested it and it works great. Does not come with a key, takes a 3/8" Hex. Back plate appears to be custom, it is 8-1/16" in diameter and the bolt hole centers are about 7-1/8.
Surface is nearly perfect with some superficial scratches and a small very shallow round mark dead in the center.
These are about $1750 new from Suburban.