May I draw your attention to the fact that that mysterious letter also appears on the 1940-1945 German made barrel of my "Lovena" double rifle drilling? http://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=213319#Post213319
Though admittedly this letter somehow resembles the "L" used for the British pound sterling, as a German I read a "Z" almost instinctively.