According to about.com it is the lot number. From that site, "lot numbers were assigned to all products including lot "501", which contained the first watch pocket."
There is more information there. http://collectibles.about.com/od/cowboyheroes/a/joecarolevi.htm
The 501s are Levis classic style of jeans. They are what people typically think of when they think Levis.
JC Penney, Maine Mall South Portland
Levi's Original 501s.
Quality Never Goes Out of Style!
levis levis levis
Patrick Levis's birth name is Levis, Patrick Cannon.
The 501s sit at the waist. The are not low rise.
Levis Levis
Ashley Levis is 5' 3".
Gasteropelecus levis was created in 1909.
Dupontia levis was created in 1908.
Persoonia levis was created in 1798.