yeah that was me, I wrote about U+237C right angle with downward zigzag arrow/⍼. I have a new blog post about some relevant Monotype documents I looked at while visiting St Bride Library in London:
it turns out that ⍼ appears before the 1972 List of Mathematical Characters in L231 as S16139! Dr. Sallie Morris at SMG group sent back to me some photos from Index to Classified Lists of Monotype Special Matrices, dated October 1966, which contains several booklets of signs from those special matrices listed above. in "Arrows & Shafts", ⍼ is listed as S9576! you can see it on the very right of the third photo near the middle, next to the big fat two-headed arrow.
astonishingly, Morris consulted someone who had actually worked at Monotype during this period—and he must be in his 90s by now!
I consulted a former Monotype employee yesterday. (He started work for Monotype in Surrey in 1945 and continued to work at Monotype with hot-metal technology until 1992 after which he seamlessly continued to work with Monotype matrices at The Type Archive in London until 2022.) I asked why Monotype designed and manufactured so many signs and symbols that were so similar to each other. It was driven by customer requests (or the customer’s customer request). If a customer asked for an arrow or a company logo, for example, Monotype would make it. Perhaps the customer was not shown the range of existing arrows to see if one would be suitable.
so it's unclear whether somehow ⍼ was significant enough to be requested twice or if it had simply been relisted under L231. at this point, whether a particular symbol has or hasn't been listed in a catalogue is mostly mere chance, let alone the long series of happenstance that led to ⍼'s inclusion in Unicode, and there isn't much inherent to why it exists digitally while its meaning is unknown. I mean just look at all the other symbols inside "Arrows & Shafts": what are these guys here? why weren't they propagated through lists and catalogues and standards?
S4182
S4253
S4521
the other happenstance symbol in Unicode's Miscellaneous Technical that I've also been curious about is U+23E7 ⏧ ⏧ electrical intersection, which was also in L231. I'm not sure if it's in this Index, and I could ask, but even if I did found out that it is, I'm not sure what further information that would give me anyway.
in their catalogue of the Morison collection, the Index to Classified Lists of Monotype Special Matrices is listed under Morison.MC.Q17, dated 1954 – 1955. I was curious to look at this earlier version and possibly to request a high-res scan, so I again contacted Claire Welford-Elkin, the rare books superintendent at the library. she sent me back the above photos of the "Arrows & Shafts" booklet, dated December 1954, which at this point is more of a brochure. the list stops at S9198, but it does seem to include all of the signs up to that one that were also in the 1966 edition. I think it's reasonable to assume, then, that ⍼ was created some time between December 1954 and October 1966!
I contacted the seller about the item, asking for its date and whether it contained S9576, and they kindly sent back these photos. By the front page, it looks like this set was published in January 1963. They look the same as the SMG's copy, so I think no updates were made for the 1966 edition. In any case, the earliest appearance of ⍼ is now verifiably from 1963, giving an eight- or nine-year period for its introduction between 1955 and then.
