Looks like a donor door - it's a different shade of `red'
could well be on that age of `TC',
also red paint fades (real bad) I have had some 5 cars in that colour, over the years (as you also say) or more to be precise it oxidise's with the weather hammering it & the general paint make up as well, as being the older type of paint!! really newer `2 pack' paints don't fade really!!
also `Rover Metal' rusting, is the other common reason to swap panels, new parts are thin on the ground or expensive now!! I had to repair our front wings on our Tomcat for this reason, last year,
NB however this Tc is still a good project!!
