looks like a tooth off a clutch pack separator plate (the metal part between the friction material disks.
It looks like part of the retainer for the boost valve spring.
Ddid the trans start acting up? Conerter slipping?
Drop the VB and look for it. It is up on top.
Do you know the history of this trans?
Maybe it broke before you owned it, and the retainer was fixed before you, and they couldn't find that piece. It just vibrated to the pan.
If you can find a trans shop open then can remove one from a spare VB.
Been there done that!!!!