Mustang Transmission part 2001

I opened the trans pan on a 2001 Mustang to change the at fluid and found a part rolling around. The part is plasitic, about 1.5 inch long (total), with a 1 inch stem (approx 0.25 in in diameter) attached to a 0.5 inch hemispherically shaped body with an oring around the body. The is a recess or opening at the far-end of the hemispheric body.
I looked for hours at the underside of the transmission but could not find any place where that part might fit. I also searched the net but could not identify the part. The local ford dealer needed to see the car, so they were no help.
The car has 90K mile and has no detectable transmission problems and has never had the at fluid changed until now. I performed all the service on the car.
Its my daughter's car and she is moving 1400 miles away, so any help would be greatly appreaciated.
Rudy

hi. the object that you found in your auto transmission is a factory dust plug which is put in during assembly and you basically throw it away as its not needed. i found one in my mustang GT 5.0 auto box and threw it away . its purpose was to keep contamination out of the unit during assembly . my manual ( Haynes UK ) explains the above . steve