.42 Berdan and .43 Spanish are really very similar.
"almost" interchangeable. (Berdan goes into Spanish, not always the other way round.)
Could also be 11mm Beaumont, but the rim diameter would tell you which.

43 SPanish was very frequent in Belgium made RB because many were intended for South America (where the 43 Spanish was king for a long time).
I doubt they would have chambered in either Berdan or Spanish for a gun meant to be sold in Netherland, but...

Chamber.......

Barrel....

barrel jig.............

I have heated the area of the barrel, approx. 2 inches from the muzzle, to a dull red with a map gas torch twice now in a effort to determine the material and in an attempt to remove the remaining material after drilling with a bronze brush pushed out the muzzle. It's not lead based, that's for sure.

I am making ever effort to maintain the originality of this barrel but I fear it will require a new liner if it's to be shot again one day.

Another choice might be to do as armorers did with milsurp rifles that had erosion near the crown, and counter bore the barrel. I've owned a few that had been counter bored down 2" or so, and they shot excellent. It saves the exterior look, and sights, but gives you a new crown down below the solder or whatever is in your bore. It will also be the cheapest fix, and save the original looks.

Counter boring has crossed my mind. However it has to be done perfectly or the gases will jet out around the bullet unevenly and throw the bullet off target. I think to do it properly a pilot would be necessary which is an issue with the bore obstruction. Maybe if all the run out could be eliminated it could be done in a lathe with a boring bar and then trued up with a piloted reamer.

The next planned step is to purchase a reamer just a RCH under bore diameter and try to remove the approx. .006 of material setting on top of the lands. After that, if successful, I am hoping the material down in the grooves will be much easier to get out.....

I also own counter bored rifles that shoot well but they are WWII type military rifles. I have talked with a barrel relining expert about counter boring the barrel and he thinks from his experience it wont work. I am not really sure why he thinks that, maybe it has something to do with the caliber or the projectile type......