Originally Posted by ellenbr
Yeah, those stamps of coupled/interlaced KR, whatever, are prevalent on post WWI platforms and are tube maker's marks where they transformed Belgian roughed bored tubes to a final product.

Too, I have seen the Heart stamp prior but it was associated w/ Emil Eckoldt.

Thank you. It is almost certainly "KK", because it is very near to the handwritten Sütterlin style of the capital letter K (yes, I do write in Sütterlin, although rarely...).

Could be Karl Kelber, of which there may also have been more than one, but the issue is controversely disputed on three pages :-D :
https://www.germanguns.com/vb5/foru...99-karl-kelber-gewehrlauffabrikant/page2

And I have checked out Emil Eckoldt, thank you. His signet was a slightly larger heart with the three letters EES inside.

Carcano