Hi , what or why are the seats rare ?
The seat pans are a particular weak point in these cars; they tend to collect water with the slightest exposure to rain or even a heavy dew; and then rarely get dried out thoroughly. The pans are also forced to flex every time someone sits on them and weren't overly strong to begin with. Between the two effects, the metal breaks. Every TR3/A I've owned has been repaired multiple times and was still broken! (Of course, I'm pretty heavy myself, might have had something to do with it.)
The early seats are older, and there weren't as many of them produced (roughly 20,000 vs 60,000); making them rarer than original later seats. For some time, only the later pans were being reproduced as well, so again the early ones are scarcer.
Another difference may be that the very early cars had carpet, which tends to trap water and hold it against the bottom of the seat.