Andreas:
It could be as Rudolf Kelber is the only Kelber missing during this period to my knowledge. But for the Brothers Kelber the rhombus was more of a "G". Also why would both Louis & Wilhelm Kelber both have their own forge type stamps? You will see Louis Kelber's mark of a jagged encircled "K" in pairs while Wilhelm Kelber's mark of a "K" encircled by a "W" is seen a single. Like the Belgians in having a registered trademark steel stamp, like "Krupp Stahl", I believe each German tubemaker may be identified by the trademark/steel recipe and also at the end of something akin to say "Krupp Fluss Stahl" there will be a bullet and then typically there's a letter, sometimes "J", and then another mark.

Kind Regards,

Raimey
rse