BudsGunShop.com - similar Glock 19 gen 4 models on Buds

With Glocks, a U in the model number means it was manufactured in the USA. A P int he model number indicates it was manufactured in Austria.

I can't see much of a difference. The 503 looks nicer to me, its a little heavier (3ozs) with a "gas nitride finish" and night sights, other then that I don't understand the difference or the pricing being the same for both models.