Engine oil caps. R has a smaller cap with the words OIL on top. S type is larger with no writing.
The alternator bracket on an "R" is a true bracket - the "S" has a heavier, solid, cast piece.
The window rubber on and R is solid rubber all the way round, and has a metal trim piece at the top. The S has a thin plastic strip insert around the rubber. On the inside of the car, the trim on an R is also made of metal, which covers the rubber - the S is exposed rubber.
The buttons on R Series door handles are chrome, S ones are black plastic.
The ashtray on the rear of the front seat has a raised section on the lid of the R, whereas the S has a plain flat lid.
The recess in the lower portion of the dash bezel on an "R" (where the steering column rests) is shaped differently to an "S". The R has a black thinner "lip", while the S has a larger black area.