I never knew the Viking Jr existed till I won an ebay auction for 4 viking stems, and got these. The Viking Jr is a smaller version of the Viking and takes a smaller threaded bowl, which I have yet to find. To show the size difference here is a picture with them next to a regular viking.
The standard falcon bowls are of a comparable size to the standard Viking bowls, but of course the thread pattern is different so they're not interchangable. The Alco bowls only fit the alco pipes, but the standard Falcon bowls fit the shorter Bantam/Oxford stems as well as the standard stems; likewise the smaller Bantam/Oxford bowls also fit the standard Falcon stems.
right bodgit and milliput
take the thread of a bowl so it fits into the junior, some milliput around the dethreaded bit coat the metal thread with vaseline then screw the bodged milliput on then off to create a thread