Oh, I just realized it also has another feature. I think it is called a 'hidden dollshead'?
I don't have a picture of MINE, but there is an AyA with something similar for sale on Gunbroker. Here is the picture of the breechface:

Here is a picture of the chamber mouth, with barrel extension/dollshead thingy:
Mine is a bit different, but basically the same. The difference, as I recall, is that the 'nub' on the barrel's chamber is more rounded top and bottom, and slightly smaller. Yet, the principle is the same.
So- am I right that this is called a 'hidden dollshead'?
thank you.