For now it seems that Gebüder Merkel had a strong sourcing connection, possibly primary, to Gebrüder Kelber, Louis, Rudolf, Wilhlem, and more than likely didn't make their tubes till post WWII, even after their October 1930 tube joining patent. Paul Schlegelmilch & Franz Schültz were either apprentices at the Brother's Kelber or actually worked there. Any Merkels with Kelber forge marks will aide in putting the overall Suhl gunmaking puzzle together.

Kind Regards,

Raimey
rse