Does anyone know for sure what it is? Best I can tell it’s 13x1?
You could try .500" x 25tpi. (Withworth Form). Many early European designs still used Imperial type threads, although, Hotchkiss, being French based, may have used Metric from the word go.
You must use precision thread gauges to ascertain the correct Thread.
Try making a casting with Woods metal. Make the casting with a screw driver in place so it will turn out easy, then measure the threads with a metric screw gauge.
My projectile was demilled by drilling through it. The drill ruined the threads. I have a good fuse, but no metric feelers. It measure close to 24tpi and slightly over .5"
I need to buy metric tpi feelers.
I’ll end up cutting threads using a lathe and best guess unless someone knows for sure and I can get a cheap tap for it.
Metric threads are measured by the distance between threads, so a simple mm scale will get you real close to the correct pitch.
If my math is correct, you’re “about 24 TPI” would be 1mm pitch (1"/24= 0.0416" between threads). Closest guess to the diameter would be 13mm (.512"), or a 13x1.0 thread. The bad news is that 13mm is not a standard (common) tap size
We ended up chucking the projectile into the lathe and figuring out a thread pitch that allowed the fused to screw in. Turning it by hand with a small boring bar. Worked great.