Looks like gravel inside.
It is a case prepared for measuring the pressure of the ammunition, not testing the gun.
The cut-out in the rim makes sure the drilled hole is correctly aligned with a hole in the chamber wall through which the gases can act upon a piston that either compresses a copper crusher or a piezo gauge. The drilled hole is usually covered by adhesive tape to prevent loss of propellant.
Agreed, however the internal contents appears to be gravel rather than propellant. I wonder if this is a dummy pressure test round?
My gut feeling is that it was originally empty (sufficient to show the features of a round prepared for pressure measurement; no adhesive tape over the drilled hole) and at a later point someone could not resist his urge to add some “dummy propellant”.
I have two very similar rounds dated 1968 and 1970. Neither have any powder (or gravel) in them-quite empty. Both look like they have been tumbled clean and both have very vague indications that they once had tape over the holes-but that is perhaps my overactive imagination. Interestingly the hole and the notch are 180 degrees apart.
I suspect some pressure test rounds got mixed in with the huge amount of surplus 9mmP that came out of Israel a decades ago. Much was battle field pickup and contained Arabic headstamped ammo and a surprising amount of foreign ammo. I suspect other scrap ammo, including pressure test rounds got mixed in and it was all tumbled clean together. During the tumbling the tape came off and the powder fell out.
I have seen a load with the “gravel” in it like yours and assumed that it was the cleaning media from the tumbler.
If you want to check for sure, put the 3 or four loose grains in a pan and put it on the stove on high and see what happens!
Thanks to all.