First off, I'm not sure of the proper terminology. So, if a call a part the wrong name...please forgive me.
The photo below shows four NR RDs. From left to right, they are Pat. '79, '80, '82 and '83.
If you take a look at the spring covers (not sure if that's the right term), where the pulley cage stop screw threads into, on the '79 and '80 the place where the stop screw goes into extends down to the bottom of the cover. On the '82 and '83 that section is a few MMs short of reaching the bottom. Close-up photos below:
I'm curious...Which one does the '81s have?
Also, The one on my '79 is cracked. I bought another one and it's a later version (this is what made me notice there was a difference). And, being anal, I would like an earlier one. So, if some one has an early version and wants to trade for a later one, that would make my day. Or, if you have one and want to trade for something else, PM me and ask. If not, I'll just use the one I have and live with my OCD.