Stuff the snark, Jim.
Considering the absolute mind-blowing minutiae that was tracked, processed, developed, generated, designed, expounded upon, etc., during the war years, no, I actually DO think it's a surprising oversight.
Go back and re-read my original post where I talk about tiers as I recall the concept being used.
It might have been a particular author's assessment of the capabilities of manufacturers -- the term tiers may well NOT have been used by the War Production Board.
But, the concept is, and this is regarding CIVILIAN manufacturing companies:
Tier 1 -- We make guns for a living, we can make whole guns for the military NOW!
Tier 2 -- We have a lot of very complex heavy industrial machinery that can be repurposed to make guns, or complex gun parts, pretty quickly!
Tier 3 -- We have light to medium machine shop capabilities. We can make simple parts now (how many screws/pin/simple stampings do you want), now, but for more complex stuff we'll need to expand.
To simply it even further...
Tier 1 -- GUNS NOW!
Tier 2 -- GUNS SOON!
Tier 3 -- Gun stuff eventually.