The script L, or Z as you refer to as did I, is the stamp of one of the tube makers of the Boys Kelber, Louis or Wilhelm, post 1927, I believe. One will find it on the tubes of upper rung sporting weapons. Pretty high serial also so I would say a larger concern. Have you checked the sides of the lugs for a Merkel monogram or something similar? Are those Birmingham marks at the knuckle?

Kind Regards,

Raimey
rse