My '81 Mark IIB has the "R"
If you google for images of the various Mark models, you can get an idea. From my short search, it looks like Mark IIB models have them. It looks like some Mark IIa models had them too, although I did see one IIA that had a "TM" instead of the "R". Looks like the Mark I models did not have either the TM or R.
So, my guess is that the Rs were added sometime around '79-'80. Others here can probably date the models more accurately...