Originally Posted by 3alarmer

...it's probably a mistake to assume any seat post diameter for a bicycle that is as well traveled as yours must be. It's had many adventures along it's way to you, and some of them might have included re-reaming the seat tube, in order to correct prior distortion from original construction or while in use.

Were the lug ovalized, the result is invariably that a smaller post is needed to fit past it. But your best bet by far is to take posts in the sizes that are close, and see how well the slip fit works. This is best decided with the interior of the seat tube, and the exterior of the post, both lubricated with some sort of anti-seize. Make sure the seat lug is expanded to where the sides of the slot in the back seem to be parallel.

But your strategy of trying different posts ought to work. If you really can get a 26.8 in there, but it feels tight, I think a 26.4 might be unlikely to be what you will need. You're looking for something that feels like a good, firm lubricated slip fit down in the seat tube, past the lug.