Ran across a pic from an old Mosler catalog showing variations of the Number 6 lock. Some have a mounting tube, some don't, some have a boss, and some use corner screws to mount the lock to the door.
I had it stuck in my head that this was the B6 model, but based on their other models like the 10 1/2 and the 5-H , 6-B makes a lot more sense!

hope this helps someone out,
Squelchtone