Assuming this ^ is what we're talking about....
MP12 barrels were only produced for a relatively short time and were in the timeframe when IMHO, from original examples Ive seen...uppers were sort of in between "very square" and "clearly round." They most definitely would not have been found on the rebated under the port door uppers.
This is the upper I have my MP12 barrel assembled on, and I feel like its the correct profile. Its what I would call a late square forge 604 upper. Notice how the left side is slightly curved at the top:
This barrel was originally assembled on an XM16E1upper that @Morg308 had that I believe looked the same. Also keep in mind with the MP12 theres about a 50/50 chance it would have been on a full fence XM16E1 lower.
^ Full fence lower indicating produced between Jan 1967-Oct 1967 Notice the curvature of the forge lines
^ Another example of the sort of rounded at the top forge lines
Compared to how straight the early uppers were (this upper is rebated under the port door)
This is an early A1 with the MP C barrel (all original, .fed gun)
Thats my case for how uppers looked from 66-69~ish and what MP12 barrels would have been assembled on. Mostly square but slightly rounded at the corners.