I just caught up with the Gachiakuta manga and I have no idea how the theory and prediction side of things in the fandom are but my theory that I believe in more and more every day is that this 👆 man with the mask in Amo's flashback is NOT Tamsy... but Regto.
It's not Tamsy because this man acts completely different and in the opposite way of how Tamsy later treats Amo. Tamsy wouldn't have given Amo the boots, and if he were that man, why would he randomly kidnap and torture her later, separating her from her boots after telling her to take care of them.
Narratively, outside of Regto's murderer and this man having the same mask, these two people aren't connected either, as in no one or anything else draws a connection between them, not even Amo herself.
(Except for one aspect: the visuals of an angel but with opposite looks and feels. One a heavenly angel, the other a demonic one.)
What I think goes on in Tamsy's mind is that he learned how a man with the same mask and ability to fly into the sphere as him treated Amo kindly, so he decided to torture her in retaliation, similar to how he tries to torture Rudo repeatedly.
And why could the flashback man be Regto?
Because he does the same thing Regto did: give a suffering child a piece of the Watchman series to help them. This would also explain how Regto got the gloves: he went down to the ground with his vital instrument and retrieved it. The reason why he was killed by Tamsy, beside wanting to bring Rudo to the edge, is because Tamsy took Regto's position as an intermediary between the worlds.
They might've known each other, which is how Tamsy knew of Rudo. In the last chapters we also saw what looked like Ground Tourism for Spherites which would point to a moderately well functioning system of going up and down between the worlds, something where Tamsy might be involved in, something were even Regto looked like he'd been involved in, too.