Honestly, the hidden joinery on a 1920s built-in at the Seattle historical society museum is the cleanest I've ever seen.
Ngl, I spent a solid 20 minutes just staring at the mitered dovetails on the drawer boxes, which were completely invisible from the outside, and now I'm wondering if anyone has a good source for plans or books focused on that specific hidden-joint style from that era.
My grandpa had an old roll-top desk with that kind of hidden work. Found a secret panel once that slid open with perfect, silent joinery, no visible seams at all. It ruined cheap furniture for me forever lmao.