1992 Mercury Cougar Not sure what this is?

Theresa, I havn't seen one of those in years. The "stopper" as you called it, is just that. It's a plug Ford used to use at the factory during production. They used that plug on the assembly line in the hole where the dipstick tube was later installed. The plugs were usually pulled out but, sometimes, on the line the guys would get in a hurry and just push the plug in with the tube and down the line the car would go. The plan being in the trans pan never hurt anything, it just floated around. The first time I found one, I thought it was a fishing bobber. I wondered for a week how a bobber ever got into a transmission. Save it, it may be a collector's item. Hope this helps.