I have a mid-late 70's panasonic Sport and I am trying to figure out the seatpost size.

I have tried several known sizes...25.8 and smaller all fit and spin even with the binder clamped down...26.4 did not seem to go in.

I have a post out of a similar vintage Miyata "Sports Model" and it seems to be the right size but is not marked (I could only insert it 2 inches to check because the rest was rusted and flaking chrome and it seemed like it could be right). I measured the circumference with a slip of paper and measured it against a metric measuring tape (I don't have access to anything more precise right now) and it came to about 82mm which would indicate ~26.1mm post diameter.

Do I need a 26.0 or a 26.2?

also, between Promax Alloy, Kalloy and Kalloy Uno (all the type with built in clamp) does it really matter?