norman(a)oclsc.org (Norman Wilson) writes:
It's unclear exactly how far back it dates. The
earliest OED citation
for `slash' as `A thin sloping line, thus /' is dated 1961; but the
cite is from Webster's 3rd.
A bit of searching finds earlier American examples in the context of
livestock brands. For example, from "Hot Irons: Heraldry of the Range",
1940:
A crude sign at a dirt turnoff will have been "painted" with a hot
iron, reading V-/ 8.
You may or may not know that the V Bar Slash ranch house is eight
miles down that trail [...]
Or in State v. Craig, New Mexico, 1922:
The calves were then placed in a corral at the Webber ranch, and
several days later were branded by appellant with A slash brand and
then turned into the open pasture.
--
Adam Sampson <ats(a)offog.org> <http://offog.org/>