Do you mean the steel Frome/springs or material?
As base/l are different in pre & post facelift but XL/GXL/E are the same across all years.
GT tombstones are unique but again same for all years ( note during 74 XL type were fitted to GTs).
Material wise it's more complicated due to trim level/year/cloth/vynil/colour.
So you need to be a bit more specific.
The driver seat is interchangeable ( get less wear & tear) with passenger one only the tilt lever is on the opposite side so is a better bet.
I've a black prefacelift base/L cover with a small tear (easy repair) £25 plus p&p if any use or can take to Cortina day in a fortnight.