The original '68 kit had low back bucket seats, and headrests were not required until Jan 1, 1969, so maybe MPC decided to skip the seat upgrade since they knew the '70 was going to be the end for the AMX model, both 1:1 and 1:20.
The original seats appear to be the same as found in the reissued kit:
Yet they tooled up the correct dashboard. I've also heard people talk about seats molded in vinyl rather than styrene in the original '70 kit. Were these seats the correct high-backs, maybe? And with the old seats still on the styrene trees?