The "lines" on this brass end cap represent another form of knurling known simply as "straight knurling", if my memories of cold metalworking industrial arts classes are correct.
Engine lathe work done at slow speed.
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David Longenecker
R.K.C.C. #CM83
RKS#1691
N.R.A. Endowment Life Member
Residing somewhere in the Ubiquitous States of Amnesia