I still think the "double duck" stamp is an intertwined J&Y but let me still search for a better image of the stamp. Otto Bock sourced the whole of the continent so I wouldn't attach much to the serial number until the marks have been accounted.

Z-M sourced with Louis Kelber tubes:
http://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=159483&page=3


This E. Kerner appears to have been started earlier that 1943 but it may have similar stamps after the bullet on the Krupp Steel stamp. E. Kerner could be another choice of the K in a rhombus.


Note the script L as typical. I don't think the Boys Kelber let anything but quality out of their shop(s).

F.W. Vandrey thread:

http://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=160489&page=2


Kind Regards,

Raimey
rse