I have the same exact drilling myself. If anyone has information as regards that rifle chambering, I'm all ears. I've also seen it listed as a "GECADO" loading.

Right now I'm close, but concerned what I've cobbled together will be too hot.

I've determined the bore is .439-.440 by pounding various sized round ball down the bore. It has odd numbered henry style rifling so just measuring the resulting slugs was useless as if you measure the resulting slug from pounding a .440 ball down the barrel you get a .433 land to groove measurement. Pushing a .433 ball down the bore just barrel marks the slug with a hint of the rifling.

Anyhow, I didn't do a casting, but tried various sized brass after making chamber measurements and right now I'm using 400NE brass cut down to 65mm, fire-formed with a .440 round ball over around 80+ grains of compressed Goex CTG. Someone that looked at the proofmarks on my gun said it wasn't proofed for smokeless. The current loading I've not fired yet is a 43 Spanish bullet, .439 dia at 370 grains, over a charge of 80 grains Goex CTG. I determined this load buy seating the bullet until I had 1/8 inch compression of the powder charge (which was dropped via a 24 inch drop tube).

I've not fired the loads yet and I'm worried that I may have too heavy of a bullet in this load. I may actually cut the noses off the bullets to lighten them and then work up to full weight to be on the safe side.

As I was told "NICHT FOR KUGEL" literally means "Not for sphere" so I would take that to imply no slugs for the shotgun barrels.