Found parts in my 904 Pan

If nothing else is broken you could drop the valve body in the car, replace the strut, adjust both bands and reinstall the valve body.

It would have dropped out because the forward band was too loose, either the lining has disintegrated (most likely, looks like lining material in the pan), the band broke or it was just plain out of adjustment.

If the lining has disintegrated the trans has to come out. I am guessing this is the case and a rebuild is in order.