My reaction is that Richard Stewart used whatever parts he could get cheaply. So there was no standard seat. He would buy end of lines and adapt the kit to suit, adapting again when the current end of line ran out and another different one was found.
Second previous owners could have changed the seat.
So You say that there are studs in the seat. So if the external nuts are loose then trying to move the studs will tell you if they go into captive nuts or loose nuts or even if they are welded in studs.