30.9mm is a Specialized only post size.
In the olden days the external diameter of a seat tube was 1 1/8''. The introduction of oversized aloonyman frames required the use of tubes with greater outside diameters, hence; 1 1/4'' and 1 3/8''. By amazing coincidence these sizes correspond perfectly to the modern metric sizes of 28.6mm, 31.4mm and 34.9mm. There are only three, three is the number of seat tube external diameters.
Back to the olden days, given that there is only one size of front mech clamp to fit only one size of seat tube why were there so many different seat post diameters? Well the answer is straightforward. High end tubing such as 531 (manganese/molebdinum alloy) had an internal diameter (and hence seatpost outside diameter) of 27.2mm. High quality, thin walled tubing. Less expensive and therefore heavier tubes have the same external dimension and a thicker wall section and therefore a smaller internal diameter, 26.8mm in the case of Reynolds 501 (Chrome/molebdinum alloy)for example.
I dont know why Specialized felt the need to invent their own size, I expect there is a very valid technological reason for it. Until they started plating silly buggers everyone graded tubing and hence seatposts in 0.2mm increments. A fifth of a mm. Which is the exact thickness of the material used to manufacture Coke cans.
(Wrong, it's 0.1mm)
