Craftsmen attached their marks pretty much as a means of keeping track of who is to be paid as well as liability. For now I think that Krupp tubes out of Germany will have a Shilling forge mark or similar tube maker's mark while the Belgian Krupp tubes will of course have Belgian tube maker's marks, which may have been removed, for whatever reason, or partially removed during the finishing of the longarm in the U.S. of A., a country that didn't have a set of proof laws. I think the following gives insight as to the cottage industry aspect of gunmaking on the continent and was similar among other gunmaking centers.

"One of the great advantages arising from this triple proof is that each class of workmen has a direct incentive to only turn out, or accept, really reliable material, for no one who has worked upon the gun is paid for his labour unless the arm passes the three proofs satisfactorily. Thus, if the barrels burst at the first proof(viz., that of each barrel separately), the barrel-maker loses the cost of his labour and the material, for he is obliged to replace the burst barrels without any indemnity. Should the barrels burst at the second proof, it is not the barrel-maker alone who suffers, but the solderer was well, who also loses the price of his labour because he had not examined the pair or barrels carefully enough before working on them. If the gun bursts at the third proof, all those who have worked upon the gun, from the barrel-maker upwards, lose the benefit of their labour; and thus, as I have said, each class of workmen has a direct personal incentive to turn out a really reliable gun."

from 1891 in:
http://books.google.com/books?id=-X3NAAA...;q=&f=false

Kind Regards,

Raimey
rse