I tried to install the Pro-Mag flip-up sight (PM138 & 137) on an S&W 15OR. On this AR the upper receiver rail is inline (co-plainer) with the gas block rail. The front sight post ended up being ¼ inch high.  I bought Yankee Hill Machine flip-up sights and they work fine. I am new to AR’s but I am quires why Pro-Mag would make sights that do not align on the same rail?

P.S. this is my first post and this forum is a great resource.

Most picitini gas blocks sit lower than the recievers pic rail, so a flip sight for them would be taller to make up the difference.  If you told the sales guy you wanted a flip sight for a gas block without saying it was the same height as the reciever, then he would send you one for a standard gas block which sounds like what happened.  

You will quickly learn to avoid Pro-Mag products.