Canon EF Lens Mount Assembly, 60mm x 60mm Plate. Used. Likely removed from scientific or industrial imaging equipment, unfortunately I have no brand or part number.
This appears to use a modified Fotodiox Pro 21mm macro spacer for the lens facing portion.
Flange distance is ~30.7mm to the flat portion, and ~36mm overall including the round protrusion. The round protrusion at the rear is ~54mm, the square portion is 60mm x 60mm.
