I think you are right about the lenses. Noticed Ermintrude had odd rear lights and tried to pinch one to make a matching pair from Ian's spares car in France. It didn't fit at all, so old one went back on. It'll be something daft like being interchangeable one way only.
Theres one type with a smooth rounded cover for the indicator and tail light, and another with a step around the edge. Bet the ones with the step are the ones that will fit anything.