Is it a PH made in Birmingham or and Italian one?
If it's Italian, it's going to be a metric thread, but I've seen two or three sizes on different rammers. You can check the metric thread in most Lowe's or Home Depot stores in the nuts and bolt section where they have fixtures that show the thread sizes.
If it is real PH of Birmingham, then the thread size should be 12 x 24 TPI.
Gus
P.S. Oh, if you know a machinist or someone who works in say an automotive repair shop, they may thread gauges or have dies that will tell you what size it is.
I would NOT order any tips or jags before you find out for sure what thread size the rod actually is. Since it looks like a put together gun, the Rammer might be Italian or English, but there is no way of telling which one someone used to assemble the rifled musket. Better to find out exactly what you have.