It's a bit odd to see this letter form standing alone, more commonly it's used in handwritten minuscule for the second letter of ττ or πτ. I don't know if it has a name.
It looks very similar to the irish ligature for &. I would look into that maybe. It’s called the “Tironian sign et”
Yes, you're right, it looks like the Tironian sign (⁊), but this does not quite explain the usage described above. In the book, this τ varation is only used for this adnotation and in Greek words. No such abbreviations are used for et (or καί) here, and the abbreviations in Greek manuscripts look different, too (I've just checked with Cereteli to be sure), so I believe the similarity is coincidental here.
I have been encountering mathematical nearly satirical articles on the internet that want the Greek letter τ to have the value of 2π. This post reminds that two tau ττ written together could be mistaken for a single π. : )
That is really weird. Two thoughts: maybe check the area of unicode when they did a more recent update--like when they added the batches of symbols for Linear A and B and the revision... maybe something odd like that got added in the same batch.
Other thought: find out the country where that was published and typeset, and figure out the languages that were spoken there. Likewise the native language(s) of the author. See if there was some sort of peculiar thing in those languages that could have bled into the usage. A historian of those countries/languages might be familiar. Just like rursus has two different forms of s in the second picture, maybe the country had two different forms of some native letter that they used and it just bled into their publishing.
Or sampi, I get them mixed up.
It's neither, I believe. Qoppa would be between π and ρ, sampi after ω (if I'm not mistaken), and the Greek words that include this mysterious character definitely require a τ there, and I don't think the shapes match. (There's no digamma in his adnotation-numbering system, either.)