You may occasionally find a PMI mags in packaging marked Federal Ordnance. These are the same mags, similar packaging, and the same part numbers as PMI. The Fed Ord mags had blank floorplates. Most of the PMI's have blank floorplates, but some have the PMI markings.
These mags can be identified by the mag catch tab on the back side. The PMI/Fed Ord mags have a welded-on rectangular tab. The USA's have a simple roll in the metal and blank floorplates. Ruger mags have Ruger marked floorplates. BUT, there have been some USA mags floating around with Ruger stamped floorplates. Beware.
I used the 30 rnd and 40 rnd Fed Ord mags for some years, and later, the PMI mags. They all worked well. Also had a few 20 rnd Rugers, which I sold, but also worked well. I have a few USA's, they work well, but the metal is thinner, and they seem flimsier. I don't know how they would hold up under strenuous use.
I did run into some Mini-14 mags marked Mitchell Arms. These had a much sharper curve than Ruger or PMI mags, and just would not function. The geometry of the mag was all wrong.
Last good source I had for the PMI mags was www.deltaforce.com