Please help me identify this broken part(?) from my 904!

Hi,
I been working on my 69' dart a little more. Last time time I spent some time on it I got the engine and transmission in but the car didn't have reverse. The transmission is a $40 904 off of facebook market place and I stupidly didn't check any of the adjustments before installing it (just took the pan off saw it looked pretty clean and threw it in the car). I finally got around to pulling the trans pan yesterday and found several things I need to correct:

• The rear band was not installed properly (I think) and was extremely loose. It seems like the band was not properly seated into the trans so it was impossible to get the necessary tension for reverse to work. This explains why reverse worked on jack stands but not with car on the ground. I think this is an easy fix.
• The accumulator spring was not there and there was also no blocker rod. I will be adding a blocker rod.
• A small broken round piece of metal fell from the trans when I was removing the valve body and I'm not sure where its from. I have looked at a bunch of videos and parts diagrams to try to determine what the piece is. I first though it was part of the front servo (see pic) but upon disassembly that part is still fully intact and present on my trans.

Any ideas where my mystery chuck of metal came from? I would love to replace everything I need to while Im in here the 1st (2nd...) time.

Thank you very much! This site is extremely helpful for a newbie like me.