Herman Miller Tandem Parts  

MidCenturyMetropolis,
Greetings to you. We have now eliminated the pleasantries.
The better the photo(s) of the part(s) you desire the higher the probability of success. Help others to help you. Your parts description is vague at best.

Here are two pictures of the bracket, one by itself, that other attached to the rail system used for tandem seating. Doesn't matter what type of shell as the mount is the same. As you can see it flush mounts to the top of the rail then the shell chair sits in the cradle of the bracket and screws in from below.

I'd suggest Graham Mancha. He's your best bet. Good luck.

Thank you Kin1117. I have emailed him asking for his advice.