I have been told the change over to USA sourced parts was in the mid-23,000 serial range. I have an all US 23,000 range gun and a 24,000 range one to compare it to for verification. No Uberti parts are to be found.
I've also been told there is no actual cut off number in the 23,000s after which you can be assured your gun is all US made. They used up what they had left from the Uberti parts days so some guns will be a mix. By 24,000 supposedly no more Uberti parts were in use.
Some of this is from a former USFA employee, some is second hand but from USFA collectors and fans. Believe at your own risk, it's the best info I have. (smile)
Dave