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.
Levis Levis
Persoonia levis was created in 1798.
Dupontia levis was created in 1908.
Xanthodaphne levis was created in 1988.
Ashley Levis is 5' 3".
Levis Swartbooi was born in 1984.