It's short for Ultra-Lux.
There is a CZ-452 Lux, and this is a variant of that model. It's main distinguishing feature is a very long (approximately 29") barrel.
Besides being a bit different looking, it gives you an impressive sight radius when equipped with aperture sights, and also makes the rifle surprisingly quiet. It's even more quiet with .22Short ammunition. Unfortunately you can't use the "cap" or CB ammo with the long barrel--the projectiles can get stuck.