The number became a brand
Back when I worked at Hattams (mid 80s to early 90s), we stocked Levi’s jeans, primarily the 555 “red tabs”, but also the 517 “boot cut”.
The numbers were Levi’s product codes. The first digit 5 signified they were for adult men’s — my recollection is that the “student” (kids/young men up to a size 81 cm/32″) sizes in red tabs were 855.
We didn’t stock women’s jeans, but they seemed to be all numbered in the 700s.
And while the sizing was multiples of inches, like most things in the trouser-world (in Australia at least), it was measured in centimetres.
At some stage in the last couple of decades, the product codes have become marketing tools. 501s in particular have been catapulted into a fully-fledged brand of their own. They used to be a specific type of button-fly men’s jeans; now they seem to be a catch-all for anything Levi’s feels like calling “501s” for marketing purposes, not to mention thrown onto promotional T-shirts and hoodies.
I think the women’s equivalent used to be 701s, but now they too go by the 501 branding, as I noted recently when Marita was out jeans-shopping.
They’ve also reverted the measurements from centimetres to those archaic inches, to give themselves some genuine American-ness. At least, I assume that’s the reason, as everybody else has switched to metric — well, apart from Burma and Liberia.
This page doesn’t claim to be authoritative, but talks about the numbering of Levi’s jeans — in particular it seems to be a list from after some of the numbers got mixed up a bit. I know I have two pairs of 504s (bought at Hattams in the last couple of years) which that list reckons are “slouch straight for women”, which I’d dispute.
Presumably internally Levi’s now have a completely different numbering system, so they can tell all the myriad of “501″ products apart.