exploded Parts diagram on Opposed Forces site... part #20 is the "basket" - called complete shaft in the parts list.. and yes, it would appear that the piece you marked in red in the picture is the backside of that part...
http://opposedforces.com/parts/legacy/us_b11/type_17/automatic_transmission/at_transfer_and_extension/illustration_3/
I have pointed out some of the grooves that need to be sanded out below.. on both the outer basket and the inner portion..( ALL grooves need to be removed.. not just the ones pointed at, btw) grooves will typically be worn in all the way around. If they are severe enough, it would warrant replacing the parts rather than attempting to "fix" them
The clutch plates need to move freely inside the basket.. when the baskets get these grooves worn in them, the plates get stuck and do not move properly.