I call the assembled receiver as you describe it the "upper"; and the assembled frame with side plate the "lower". KISS.:cheers:

I pretty much agree with Karl. I have always liked the term "cannon" as the collective description for the whole group that constitutes the "upper". It is simple and elegant.
Ron

Karl, if you were to include the ejector in your discription, i'd call it a luger "top half assy". TH

Quick update for anyone else who may have my same question in the future:

I did have to shave down the rails as Karl suggested.
Before I did this the barrel extension would not travel fully into the frame.

I had to remove more than expected and it was not something I enjoyed doing!

So the answer to my original question is:

A 1906 receiver can be made to fit but if you have the option it is best to go with a 1908 spec part.