I don't think it's a tag or patch. I think it is a type of fabric that is put in between the pieces of leather and the lining which I think is why the leather is showing on the rough side versus the oiled side.
I know this is an old thread, but I found this in one of my old boxes. Some chipping on the front edges, but neat finding it serial numbered!
1998 Fleer Ultra Pedro Martinez Platinum Medallion 041/100:
Found this in one of my old boxes. Some chipping on the front edges, but neat finding it serial numbered!
1998 Fleer Ultra Pedro Martinez Platinum Medallion 041/100: