The Levi's 504 jean is no longer in production and has been replaced by the new 516 jean. The 516 is an improved version of the 504 and is available in the same fabrics and washes as the 504. It's basically the same jean but more comfortable.
The 516 is a true straight, straighter through the leg and has a slightly wider leg opening. The 516 is a bit looser in the waist (but stick to your normal size) so they're more comfortable.
We've found most customers prefer the 516 fit and you'll find they even look better on.