I think Smokey is right. US Army "McClellan Saddle" there must have been a variety of tag styles because most I found on line were size tags, mounted on what would be the pommel on a western saddle. I think for a period they were not sized but one size fit all instead. The tag you found might have been off of that version and a very early production model. The later ones had "US" tooled into the leather of the stirrup covers.
I had one once upon a time and I think mine had a size tag also. They were notorious for being uncomfortable and weren't used by the folks that had them. We had an Army Depot here that trained and broke horses, mules and rescue dogs for the military. After the cavalry became the tank corps all their saddles became surplus and were either sneaked out or burned along with hundreds of 30-40 Kraig carbines.
Just about every farmer in the county had one hanging in their barn. I helped a farmer dig a large crain out of the mud and he gave me the saddle in return.