Not to split hairs, and you are absolutely correct in that it is a variation on the NEARLY-same upper, but it's worth mentioning that this variation uses a totally different material for the receiver of the pistol. And a different grip angle. With a different magazine, the both of which must somehow result in a dissimilar feed angle given the differences.
Sure, Ruger and the Fed Gubbmint call the upper portion with the serial number as the "receiver" but it's the lower that has all the trigger and sear guts in it.
The 22/45 is accurately described as a variation on the Mark II and III pistols...but in it's actual working form, it's very
different. More so, than it might seem.
I think if you told a salesman that you needed a Mark II and he slid a 22/45 over the counter... he'd think he was right, but I'd -KNOW- he's all wrong!