Tange was the only one of the big four tubing manufacturers to employ a double butted seat tube on a regular basis. The double butted sets, #1,#2 and #3, all used the same double butted seat tube. This appears to have been a conscientious decision based on cost and logistical considerations, as it allowed manufacturer's to stock a common post for these sets (along with the #5 plain gauge tubeset and the #102 single butted hi-tensile tubeset). The only exceptions in the 1970s were the single butted #4, the double butted #101 hi-tensile set and the lightweight Pro tubeset.

Only when Tange introduced Prestige did they succumb to the weight weenie demands for single butted seat tubes, offering builders a choice of seat tubes. Even the 1980s, the Tange 900 and Inifinity tubesets would employ the standard 0.9/0.6/0.9mm seat tube, though in a seamed version.

During manufacturer, it is fairly common to experience distortion of the seat tubes. This can cause manufacturers to use smaller posts. Some manufacturers routinely ream/hone to correct this and in some cases will go one size larger, to ensure the tube is round. It's also easy to become too aggressive with this operation. Consequently, it is fairly common to see posts that vary +/- 0.2mm from the nominal size.

There are also cases where manufacturers will request a custom tube. Centurion did this with the Turbo and Ironman models. Despite the Tange #1 decal, the tubesets are a custom blend and typically are fitted with 27.0-27.2mm posts.

The bottom line is that it fairly common to see posts that +/- 0.2mm from nominal, after allowing for clearance.