I THINK the 17.4 and the 18.3 are the barrel chokings in metric.
The other stuff, I simply don't know.
You need to have a gunsmith take a look at it before you fire it. It may be old enough that it's for the old shorter metric shells.
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.