Hey there Mick, I tossed the clamp seat base away long ago.
Mate, I have set my seat up in this way, Using a fold down seat with a S/S swivel mounted to the seat , I then worked out where the best position for the folding seat to be fitted to the tinny seat. I used two pieces of Rope Sail Track about .600 long and Pop Riveted that to the Tinny Seat, fitting ,snuggly around the S/S swivel of the foldy seat. I fitted two stops one at each end. I can now adjust the foldy seat if I am in the tinny on my own. My weight holds the seat steady with no movement . This has worked like a beauty, We roof top the tinny allround the state, no problems so far.
Cheers,
Brush.