That would be a sprag. I wouldn't reccomend you trying to rebuild it yourself since your not familar with them already,
I'd take it to a local tranny shop or someone more familar with rebuilding transmissions, I'm sure there's other damage there and just sticking another sprag in is just asking for trouble.
Things you can do while its being rebuilt would be flushing out the converter and trans cooler lines and the cooler it self BEFORE you go back with the rebuilt trans.
A spray can of brake clean or a can d natured alcohol will work good on the cooler and lines just spray the brake clean in the lines or pour the alcohol in and use a rubber tipped air nozzle and blow them out till the solvent runs clear out the other end.
Also you'll need to clean the valve body as well but the tranny shop should take care of that if you have it rebuilt.
Goodluck, Charles