John Moore asked for info on this tool's commonality, etc. The tool's
were apparently fairly widely used and are pretty common. On most of
them, however, the markings have become pretty illegible. For John to
have been able to make out as much as he has on his, he may have one of
the  scarcer ones still in fairly decent condition. There is a slightly
different version on which the arc has a rougher form; that type
apparently predates the patent and is much scarcer.
The tool was patented by a Thomas R. Cook; he assigned part of its
rights to George A. Topp and two others.

I have a copy of the patent. If John (or anyone else) would like a copy,
please ping me directly with a snail mail address.

Best Wishes,
Bob