Originally Posted by Minnewaska
Very common adapter, as linked by others. Most often used to have a 1lb tank on hand as a backup if you empty your large tank.
If you are trying to store the 1lb tank with your large tank, the longer thinner torch tanks sometimes fit better.
I found the adapter at West Marine a few years back, and added two small tanks (carried in an outside locker) after I ran out of propane on the big tank one cold morning just as I was about to make breakfast/coffee. Another boat nearby had the adapter and a small tank, and that saved our bacon!