Maybe. It all depends on the amount of time and money the framebuilder wants to take in finishing/re-sizing the inside of the seat tube. Ideally you want a nice sliding fit between the post and tube. Anything too tight and you mar the post. Too loose and the post is not not properly supported along its insertion length.

During the frame manufacturing operation there can be various build-up on the inside of the tube, including scale, flux, primer, paint and in the bad cases, big blobs of brazing mat'l. If the manufacturer can properly control the manufacturing and minimize the amount of build-up on the inside of the seat tube, he will reduce his costs to finish the inside of the tube and maximize his profits. This is obviously critical on mass produced frames, with lower profit margins. The easiest way to minimize these costs is to simply insert a smaller post.

In the case of good frames, most manufactuers will hone and/or ream the seat tube to clean it out and optimize the fit. This normally involves re-sizing the inner diameter a little over its original dimensions and verfying with a gauge (or an actual seatpost) of the intended post size . In the LBS where I worked, we would verify and if necessary, re-size the seat tubes on bare frames. If you got it just right, an uncinched post would not fall into the frame and pushing down on it would compress the air in the tube, making the post pop back up.

I've even seen cases where framebuiders have apparently enlarged the inside diameter on a thicker seat tube to match the inner diameter of a thinner seat tube on a more expensive frame. In relaity, they proably just spec'd a mixed tubeset with a higher graded seat tube. Either way, this adds costs and results in a larger than expected seat post, but it is offset, to some extent, by not having to stock as many seat post diameters.

In general, quality standards have improved over the years, and todays fits are probably better than the old days. And better frames should have better fits. Depending on the age, level and manufacturer's practices, you can get quite a range.

The bottom line is that the inside diameter could be over or undersize for the tubeset. Ideally you want to perform trial and error with a number of posts, working up from a undersize post until you get the correct fit. However, this is not practical and the alternative is to measure the inner diameter with either a caliper or seat post gauge. The problem with these methods is that they cannot measure very far down into the tube and will give false readings if the tube is pinched or pried open. So you want to ensure the sides of the cinch slot are parallel and that the inside of the tube is smooth and clean. Then go with the the closest undersize post.

In the case at hand, given the assumed age and level, it is hard to say if the tube was re-sized. During the boom, many operations were cut and quality suffered in an attempt to increase output to meet heavy demand. However, the inside of the tube certainly looks it recieved some refinishing, so it is likely 26.6mm, but it could also 26.4mm.

I realize this is a lengthy explanation and that it throws some complicators into the practice of using seat post/seat tube diameters for identifying tubesets on unmarked frames, but we must remember that this practice is only a guideline and is still the best tool at our disposal.