pommie john, Which one are you saying is rare? All the RTs and RSs came with the 32/11, which in the day were called the Autobahn rear end as with it you got the highest top speed. The 33/11 was also very commonly found on a lot of other bikes, including some R90s and R100s. It was also a factory option for the R80s of 1978. I know of at least 3 R80s near by with the 33/11 and a lot more with the 32/10 and 2 that have the 37/11 which was called the Alpine rear end due to it being very popular with those that did a lot of mountain riding. In the mid 70s thru early 80s, you could order different rear ends on the bikes, if you were willing to wait for it to get shipped.
But, you could also pick your bike up at the factory or one of the dealers in Germany, back then as well. You just had to make the arrangements thru you local dealer and pay up front for it. I did just that in 1979 for a brand new R100RT, I got to ride it all over Europe with my wife, then take it back to them and they packed it up and arranged the shipping to Montreal where I picked it up again and road across Canada to home. They don't do that anymore, which is too bad. It was a great arrangement for a minimal extra cost. I think it was an additional $250 back then. Would probably be around $5000 now.