The earlier guns, generally* in the 150 serial prefix, are called lowback due to the way the back of the frame (cylinder frame and grip frame are a continuous casting) drops lower where it meets the backstrap than on the redesigned ones. In the two pics immediately above, my boxed gun is a lowback and Armybrat's is a highback. The round top of the lowback grip panel versus the pointed top on the highback grip panel is the easiest way to spot the difference if the subject gun still has factory grips. With aftermarket grips, you need to look at how much of the hammer hump below the spur is visible above the frame (more hammer showing on the lowback).
