The mount was designed around USGI prime vendor specs in a 2008 market reality where the parts from 8-12 large name brands of rifles used those parts.
Those create a.124+.004 -.000 rear groove width. They create a .8615-.8633” od tolerance in general.
The nato print allows .13”-.02 for groove width, and .864-.004 for diameter, but the USGI parts don’t typically fall all over this broad range. The european Nato parts typically imported are the parts that have thin grooves and large and small diameters less suitable for use as mounts.
There are also cheap look alike parts on market, that have different looking slots, or that have 9mm compatible bores rather than approximately .338” minor ID like an actual A2. Unfortunately there are a lot of companies that have gotten into the market and the quality of parts and their dimensions varies a lot more than it did in 2008.