This is a Federal Ordnance rifle built on WWII rifles. I also have the same marking on the side of a 2 million M1. These were later 70's or early 80's built rifles if my memory serves me correct.
These might have been earlier in South El Monte, California. There is no need to worry, it is NOT anything to do with US arsenal manufacture other than who made the receiver and attendant parts. Mine came with a beautiful California claro walnut stock, lockbars and a new barrel. I bought this thing long before I knew better, but it shoots well although not famously tight groups.
Mine was apparently in salt water for some time as the metal below the woodline is severely pitted. I found a 2 mil receiver one day at the South Store and bought it for a transplant of the usable parts.
Good luck with yours. I would term it shootable (with knowledgeable inspection) but of little worth in collectors' circles.[/I]