what part is this called?

i was in the middle of A904 trans overhaul, and was having trouble getting pump to seat with allowable end play. thought I would install the band strut and servo to hold front housing to get to install. but installing the "servo" I guess it's called. the small metal ring broke, and I used trans gel, I must of cocked it. not in my overhaul kit that I can see, anyone know what it is exactly called so I can search for one on the internet parts stores? pics are attached.......oh and when installing the tf-2 trans shift kit, this part is NOT modified , Correct? only saw info on the other 2 servo pistons, which I have billet to install instead.

A904build5.png

transservo1.png

transservo2.png