I read once that if you tried to build WWII-era aircraft according to specs and mechanical drawings, they wouldn't fly. The real design was found in the dies and jigs that were adapted after many iterations and test flights.
You might have fitting issues with what I like to call "slide ruler error". How things came to be and are, are often at times not documented. Dimension and tolerancing is lost over time too.
Statistical quality control was not widespread at the time outside of a few enlightened industries. The Army was much more likely to buy a lot of parts where they could expect most of them would fit, rather than having all of the parts in statistical control within the tolerance limits.