I think you can get your answers from PTR.
“PTR makes many of the parts for its rifles here in the United States, however, for economic reasons, when high quality surplus military parts become available PTR does make use of those parts as well.
US made RCM trunnion for the HK91/G3 series. Machined completely from ordnance grade 9310 enhanced steel and heat treated to exact HK specifications. This item is the best USA made trunnion on the market. Machined to spec and are to custom ground where the barrel presses in, making the install painless. There is no finer US made trunion available.”
I currently have a clone made by the infamous Century Arms, it runs flawlessly. I would buy a PTR 91 in a heart beat.