Argumentum ex silentio. You'll have to do better than that.
To make an argument from silence (in Latin argumentum ex silentio) is to express a
conclusion (emphasis add by NdF) that is based on the absence of statements in historical documents, rather than on presence. - Wikipedia
You are the one who will “have to do better than that”. I did not say absence of documentation was proof for a reasoned analysis. I said it was an indictor. Meaning an indicator that you should look at non-specific documents for clues. Those are what I based my reasoned analysis upon. Why would Stoner be thinking about designing a rifle with a receiver that could be assembled into weapons of different sizes and calibers? The biggest influence in the country for a new military rifle wanted to rid itself of weapons of different sizes and calibers by adopting a one size fits all rifle design in a single caliber to replace them? That would be the prize Stoner would most want and focus his attention on, not any by-product of the design that enabled modularity. Thoughts of modularity probably came soon after initial design but not during initial design period.
If the components are in spec, headspace will be in spec. Parts manufactured outside of tolerances causing problems is certainly not a design flaw.
I don't know of any designers that would encourage a assumption that Parts will always be manufactured within tolerances. That is why "fixed" headspace firearms have headspace gages to ensure correct headspace when doing a reassembly with new parts. In fact, situations can arise where two parts are within in specification but each is at an opposite extreme of specifications with the result that things don't just work the way they should.