Exactly right -
A sawtooth setter.
Flip it upside down from the picture, and place over the teeth
Adjust the threaded "anvil", which rides against the blade of the saw,
To set the amount of kerf you create when you bend the teeth
With the piston/lever apparatus.
Though there is no scale, this one looks
Like it might have been designed for one of
The old two-handled "misery whips".
Doesn't look like a handsaw unit to me,
Just 'cause I still have a couple, and files,
And sharpen my saws that way.
isher