[KICK4426]

I recently shot a Glock 21 with this set up. It was GREAT. I am looking to purchase a set for my 21 but, I keep running into two versions of the same sight that seem the same. I hope someone can tell me the difference between the following two descriptions.

AmeriGlo +0 Ghost Ring 3 Dot Night

AmeriGlo +1 Ghost Ring 3 Dot Night

What does the +1 or the +0 mean?

Thanks