Looking again on second glance ... and this is the key. At that point in Stargell's life, he gained a TON of weight. If you can find a photo of him coaching first base you will see that.

I've seen one of the Stargell Braves game used jerseys and the thing is HUGE! No way was Willie fitting into a size 50 in 1986, esp when those things were snug to begin with.

And there lies your answer on this one. That's an MiLB jersey with a #8 and a nameplate was added most likely. Can't tell for sure without holding is and checking alterations ... but my guess that's an MiLB Bucco special! $50 jersey ... maybe! ; )