Yes, I missed it by a year - March 1929. Merkel must have used the fastening process before they filed the patent. Tube effort by one of the best, Wilhelm Kelber & it has the interlaced J&Y for a mechanic that Günter & I are chasing. The 16867 could be a number for Wilhelm Kelber or for the source of the Böhler tube steel. Is there not some proof data accompanied by a Crown over N on it somewhere?

Kind Regards,

Raimey
rse