This is Medieval Sleeve Nr. "I lost count".
More precisely, this is Medieval Sleeve Nr. 3-b-"I lost count of the underarm gussets".
It looks rather weird.
But it works.
The triangular gusset goes to the back seam, as seen previously. The other gusset is the final underarm gusset and goes from the very pointy point at right up to the bend at upper right-to-middle. That bend itself is there because of the changes I made to the front armscye.
(And do not worry if you got lost. In flat form, I can barely wrap my head around it myself. That's probably why it took me so many attempts.)
To provide you better understanding, here's the much more - at least for a modern seamstress/tailor - intuitive Medieval Sleeve Nr. 1b:
The width in both cases is the same - 29 cm. So is the back seam gusset, 9 x 9 x 9 cm. The difference is in the height of the sleeve cap and its additional width which results from the changes to the front armscye. I think it's kind of what happens in the back of the Irish Moy Bog Gown - a milder version of it; except that in my sleeve it happens in the front. (There's a pattern for the Moy Bog gown on this site, under "Undergowns".)
I have no idea if this construction of mine is period-correct or not. But it works. And that's most important.