These measurements taken with Mitutoyo digital caliper. Receiver ring: 1.352". Hole spacing: I am not going to try to eyeball the centres of the holes, so I will start a screw into the holes and give you the outside-outside and inside - inside dimensions. First hole from front of receiver ring: .273,.136. Receiver ring holes: .993, .745. Rear bridge holes: .733, .487. From extreme front hole to extreme rear hole: 5.23, 4.98. Screw diameter: .135. These measurements from a B prefix receiver late enough to not have been d&t for peep sight. You could also get a one piece four hole base and copy the dimensions.
