GD Yankee(that term is used down here & I assume you know the definition):
As I've said before, many before me have provided very good sources, but thanks.
Note the "M" common stamp with the following E. Funk drilling owned by Mark Baumgarten, whose father Robert Baumgarten penned a few articles for GGCA:
For now I think both were some type of sub-contractor to the tube/pipe maker who performed some type of psuedo-final task like polishing the tubes, cleaning the ribs or something of the like. If anyone has any other German examples with the "J", "L" & "M" stamps, I'd ask that you post.
Kind Regards,
Raimey
rse