At the end of the XX notation, the lot number of the zippered version of the 501 was changed from 501ZXX to 502. Just as the 503BXX, the button fly usage size, was dropped to 501, the 504ZXX was dropped to 502, the zip version was also dropped. This is the one I bought with 60% of the color remaining in the used.
top button (back)
||seat||front rise||back rise||thigh||inseam||opening||total length|