It's been too long since I reassembled the MPS. I checked the vacuum and found a sloooow leak (10mmHg over 60sec). Looking for a source of the leak I have taken the Part load stop plate off and the diaphgram. There is another metal ring present and I do not recall wether it is mounted next to the case or next to the partial stop plate. Brad, I don't find it mentioned in the article. This is an 049 MPS.
The diaphgram looks new. No green corrosive action.
Thanks,
L. McC

I'm pretty sure that it goes next to the part load stop plate. So, during reassembly you will insert the diaphragm, the spacer ring, then the part load stop plate. I'm sure somebody else will chime in and telly ou for sure.