Great questions....and the debate rages on.
Here's what I've been able to put together...
In '67 we had the RR-1. By '68 this was replaced with the RR-2. Now, there are records that the RR-3 was the latest model in 1971. Did it debut in '71 or earlier? I don't know. In fact, I have seen more RR-1 models than the 3!
If it did debut in '71, that gave the RR-2 a three year run. That's about right considering the number of examples we have in the club. What makes the RR-3 even rarer is that it was superceded by the RR-4, also in '71. So how long could the 3/3H been on the market? 12 months? 18 months?
The 4 is still an expensive machine, and still fairly elusive. When defrost was offered in '74, sales really took off. 4D's are the easiest models to find of the 'early look'. 1975 is considered the tipping point for microwave sales (and the 4D was still sold), and from that point on examples are more plentiful. 1976 saw the introduction of the Touchmatic series with the RR-6W and away we went! -Cory