Sorry, my tape measure is missing a metric scale. But, you have your choice of 3 nice Japanese clamsells. Probably made by Towa or Tasco. All are in very good shape, nice felt padding, and function perfectly. The mounting section is the standard raised flat 'plate' of the type: an 'elongated rounded diamond' design with two bolt holes not exactly 2.25" apart.
The smaller one fits a Tasco 60mm just fine; comes with two bolts and washers. $20 +$4 shipping.
The middle size measures 3.25" inside diameter. There is a nice standard camera adapter on the top. $25 +4 shipping.(Sale Pending!)
The last of out three contestants is a beefey 5.75" inside diameter. $25 +5 shipping.
Local pickup, by mail, cash, money order, personal check, Paypal. Credit cards accepted at Paypal. Will ship either Fedex or US mail Parcel post.