Hi Buckstix,
I had seen that rifle as well.
Adolphe Jansen (sometimes Janssen or Jansens) is a well known maker/retailer.
The firm was active in Bruxelles and was registered in Liege from 1859 to 1958.
He was gummaker to the King (of Belgium of course). Located rue de la Madeleine, 27.
MASSART Alphonse had patented in 1902 a method of joining barrels.
Other people have had their hands in this nice looking double. There is an IR mark that I can't trace.
You are right for the caliber, .472 would be land diameter. There are not many rounds that would work.
A remote candidate would be the 12.17x44R Remington.
Best regards,
WC-