I have this one that is supposed to be a key hole saw. I use it for a restricted area hacksaw. Somebody posted another one with some info - he said mine was like 1890's?
Yours is a "pad saw." The patent date, I believe, is for the cast iron handle (Aug 1877). If you check the internet, you will get confused since these are mistakenly lumped together with keyhole saws and jab saws and drywall saws.
The photo is of an original style pad saw, with its wooden handle and two screws that hold the blade in place. What really distinguishes a pad saw from a keyhole saw is 1) replaceable blades that also allow for different tooth patterns, and 2) the blade can be retracted almost fully into the handle. My rosewood handled pad saw dates from the first quarter of the 1800s. It has an oval shape and ferrule where the screws that hold the blade are located. Later this was simplified to use a round ferrule.
Can it do keyholes? Certainly, and well. But the interchangeability of blades and the ability to adjust the length of the exposed blades make this a very different tool. The teeth of the blade can also be protected from damage or bending in a tool box when the blade is drawn back into the handle.
The metal handles I have seen don't allow the blade to pass entirely through the saw.
