On the law side (I'm happy to admit I'm not a lawyer), as I understood it there isn't such a thing as "Section 5" component part, just a Section 5 firearm. "Once a S5 always a S5", in my limited knowledge, only applies to complete firearms. Thus if you dismantle that complete firearm into barrel, action and receiver you now have a pile of S1 component parts. Can't find a reference in the 68 Act, so I may be wrong... on the flip side, there's nothing in S5 which specifically covers component parts.
I think the theory is that these L1A1s are imported as S5 firearms, dismantled to their component parts, then (re)assembled from those components without any "readily convertible" parts which could be used to convert it back into a self-loader. So things like gas blocks and ported barrels are a no-no on the finished product because those would make it "readily convertible".
If it so happens that the rifle assembled from parts stored in a parts bin coincidentally comes together with all matching numbers, well, coincidence is a lucky thing isn't it.
