I'm visiting Seoul, and went in to look at the watches in Nostime -- they build custom straps, including this for my fixed-lug '56 recase. PS. Shim's photo, but I have no relationship with them beyond being a satisfied customer
Certainly a more secure solution than the usual flimsy clips you find on open ended straps. The permanence of it would drive me crazy though!
I didn't ask specifically -- my wrists tend to swell and shrink as I move between the tropics and more temperate climates -- but my impression was that they were pretty flexible. Not sure if they cut it out of fresh leather tho...