+1,A 31.8mm seat post also allows using a second 31.8* mm seat tube binder collar directly around the seat post itself,

I've done this removing the lip edge [Emery cloth around a dowel), on my 2nd QR lever collar on my folding bike seat post.. You can of course use a Non QR Collar.

*1.25 in = 31.75 mm


BTW seat tubes, if butted are butted Once and that is thicker towards the Bottom . (not twice .. that is the double butting for the other 2 frame tubes for the welding joints..