Hey Mike,
Its good to see another member falling in love with these machines! Here is the what Cory and I have come to the conclusion on.
The color chart -
Outside Case white/light grey - original RR - RR6 - up to '76
(I may have seen an original RR9 that had a white case too)
Outside case brown/dark grey - anything after 1977/78
Outside wood case - for most units before 1982, the only wood grained cabinet was the RRL10A (maybe RR10) that I have seen; may be wrong, but I haven't seen too many.
You will notice there were two versions of the control knobs. One is the early style like Rick's that are back lit. Those are used up through the RR4 series. Starting with the RR7, you got the revised 'dial' knobs, like yours, with a bit more chrome around the bezel.
Cory picked up a few more models a few weeks ago (an RR8A!, RR9TA series 1, and the RRL10) that we have yet to clean. Don't quote me, but we many be getting close to 20 units. The site really could use an update.
If the guy cut you a deal, then I'd say go for it - from his auction photos, it sure is a looker. These things will eventually dry up one of these days. Wish you luck!
Ben