Hmm, I think the pressings or ribs in the pan change slightly too.
I think 50-52 then 53-55, but I'm not 100% certain, and its very minor changes in the rib design. The physical pan remains the same size. There were more changes in the seat runners and how the seats attach to the pan - also standard models kept the 'wing-nut' type seat fixings till later on too.
52deluxebug wrote: