@ChemProfJSpencer Cone Mills made the denim for levis 501xx exclusively from 1922 onwards. The Cone denim used back then had a lot more character, such as your neppy feel with slubs and loom chatter. The denim changed three times, once in the mid 1950s (a lighter blue dye it seems), mid 70's (sulphur dye technique was briefly introduced) and again in 1984 when the selvedge disappeared and the denim became more fluffier/bulkier, but had pretty much lost all that character by then. (As for the denim being a flaw, l've not seen evidence to support it).
The introduction of repros from the mid 1990s aimed to rectify that, trying to get denim made that replicated that character. And here we are 30 odd years later with an array of manufacturers and different denims doing just that!
Hope that helps?