No answers for you yet.... I did a quick search amongst historical sewing blogs and that style is unusual thus far.
No bagginess or fall at the front
No fly or placket under the buttons
Is it a woven fabric?
Also do any more learnered xmarkers know what the loops at the top of the waistband are for? I've never seen that in men's underwear, only on outer garments (usually for children) to attach a bodice.
I'm throughly intrigued and excited to learn more.

No postcard so here's a Kookaburra in my garden
image.jpg
(It's the wrong way as I'm downunder -thanks GrainReaper