You need 7 US parts on a HK 90's series gun. In looking at my PTR here are the parts that I "think" are US parts and in reading some of the other postings Century will not tell you that specific parts that were used on your build.
Parts I think are US made:
3. Muzzle attachment, I think but it's not marked and it doesn't have the spring on it.
4. Hammer, C marked
5. Sear, C marked
6. Charging handle, C marked
7. Grip frame
Parts that could possibly be US made:
3. Trigger frame (I'm pretty certain that it's US made but can't say for sure).