An eprouvette is a device used by ordnance departments and armories to test the strength of gunpowder.
A carefully weighed quantity of powder (charge) was placed inside the device, followed by a standard weight shot. The charge was fired and the distance the shot pushed the gauge was measured and compared to the expected standard distance.
On these examples the powder gets inserted into the vertical tower that is capped by the lever connected to the gauge.
Not Ian’s video but here’s a [2:56] video on them. https://youtu.be/qOXoFyKvMHo?si=
I like the use of the flat spring system for that reason. Very minimal moving parts. Plus it’s mostly stuff a gunsmith of the day would have at hand.