I mean no disrespect to wood, people who work with it, or people who
measure it's moisture content.
I googled "wood moisture measurement".
As an outsider, I get the impression that measuring wood moisture
content is, as described, a crude, inexact process.
It is likely that, for most uses of wood, accurate and precise
measurements are not needed.
The reference process is: Weigh a piece of wood, dry it, weigh it
again. That is probably reasonably accurate and precise.
The field process seems to be: Make an electrical resistance
measurement between a pair of pins driven an unspecified distance into
the wood, then look in a table to get and idea of the moisture for the
type of wood. Perhaps, use partly insulated pins to measure at some
(incompletely) specified depth.
There was one google hit for someone who simply drilled a couple of
spaced holes in a block, and drove a pair of nails through the holes
into the test piece, using the block as a depth stop. Then he simply
measured the resistance and looked up the moisture content.
As pure water is non-conductive, all the measurements seem to need
calibration for the salts/ion concentration of the specific wood.
If I were still in the invention business, I'd develop and patent an
accurate, precise microwave wood moisture measurement device, but I no
longer care.

So... The thing in the picture (2983) holds a couple of pins. The
robust device is bashed with a hammer to push the pins into a piece of
wood to an unspecified depth. If or when the pins bend or can't be
removed, they are replaced. There is probably some insulation on the
pin shanks so there is a reasonable chance of measuring the resistance,
then pulling the whole thing out and re-using it a few times. It now
all makes sense. I'm happy.