We don't know exactly which boat this seat is from, it was left over and passed down over decade(s) in our garage, and now we are cleaning, and would rather not wastefully throw away.
The leather looks ok, but there is rust on the strap button to hold the seat folded down.
The post diameter at on the seat-side, measured with dial calipers, is 4 cm inner diameter and 4.9cm outer diameter. Base is just under 8" as pictured.