I was looking at my 19M last night, posted above it's the one with no "M" on the barrel.
When I received this it was missing the backstraps and only had two magazines and one of those was a gen 4 or whatever you want to call them. Glock sent me the extra mag and backstraps.
Looking more closely at the " Gen 5" magazine that it came with, while it does have an orange follower, the floor plate is that of the previous gens. I know, minor stuff but it's nice to get the stuff with the pistol that you are supposed to.
I am guessing they just have extra parts left over and are putting these guns together as they can and shipping them out.