That is normal.
See this thread: https://www.allfordmustangs.com/forum...n-problem.html
"Actually it is an pre-assembly oil dip stick plug. The transmission is built in a different factory. It is shipped without the transmission dip stick in place. The plastic thing that looks like a fishing bobber keeps the debris out during storage/transport.
When the car is assembled, the dip stick is inserted pushing the plug into the pan.
The plug doesn't hurt anything. Simply throw it away.
Now, if you find the plug in the pan, this likely means that the transmission has never been serviced."