The Crown over the R indicates a rifled bore. There was also a maker's a mark that was a backwards F joined to an R with a crown that was used by Renkins & Son. The 12 over C in a diamond would date the gun between 1898 and 1924. All the proofs you described are Belgian. The gun sounds very interesting. It sounds like a well made gun with German barrels.

If you have pictures, please post them.

Pete