Tensei Shitekara 40-nen. Sorosoro, Ojisan mo Koi ga Shitai. Nidome no Jinsei wa Harem Route?! - Vol. 4 Ch. 20.5 - Volume 4 Extras

[MangaDex]

 

[zHIST]

So that's the reason the manga turned into the fan service fest that it is now. Well, probably for the best? Otherwise it would be a rather generic isekai just with an older protagonist.