Seat covers are not side specific as far as I know.

They are different when it comes to 73/74 because they have a vinyl back so it will be different than the 70/71 plastic back seats. You know that though.

It appears the seat tracks are different 70/71 vs 72-74. Also the 70/71 had different inside tracks left and right but in 72-74 the inside track was the same but the outside (with the handle) was different.

That is strictly by part numbers. It could be that they will interchange but something insignificant was changed so they got a new part number. The repops all seem to be say 70 through 74.

Hope it is some help.