11/16 OD Toilet shut off
Here is what I determined after some research

Some (many) people seemed to think that this was caused by water freezing in the pipe and expanding the tubing but others debunked that because it tends to split the pipe and there would be other evidence of the freezing. I also don't believe that it was a freeze cycle.

Pipe and tubing sizes were set in the early 1900's BUT apparently Westinghouse supplied an 11/16" pipe which was more commonly installed on the west coast but can also be found throughout the country. Some plumbing supply houses carried fittings through the 1980's until they were considered obsolete and unnecessary.

The problem was solved in a couple of ways:


use a swage tool to sweg it out to receive a 1/2" pipe soldered on to it
have couplers made out of copper stock on a lathe