What is this tool?

[mklotz]

Don't know how a handled striking instrument would calibrate or deliver consistent impact, so anything like durometer is out.
I'll ante up the ball holds dye and the nozzle a needle valve. Depth regulated by the knurled nut...
Crude Low Resolution Tattooing?

It may be that the exact length of the sample may not be important. Say you only want to know the growth ring widths for the last two years. Any sample that's X units long is sure to include the last two years.

Also, now we now that the test medium is a solid, we know that the scale on the handle is not to measure immersion in some gas or liquid medium. That strengthens the case that it is used to measure the sample itself once obtained.