This might not be exactly what you're looking for, but a good friend of mine (down here) has a 669 and I have a 3904. One night, before my 3904 got "customized" a bit more with a nice new finish on the frame we were parts switching just for fun. I believe we've fired the guns like this and they worked, although it would have been quite a while ago if we actually did it rather than just playing around with them in this configuration.
As they were then:
Switching around the top-ends:
The 3904 -- which gets out more than you might imagine -- as it appears now: