You're exactly right about the adhesion, and the yellow paint must be a later respray. It is badly orange-peeled in spots. The primer probably isn't original either, since it would be impossible to remove the all the topcoat without removing the primer as well. I'm frankly guessing about 26.8 seat post: 27.0 fits in but it is very tight past the first 1.5". Could be a build up of crud instead of the smaller size. Regardless, some Reynolds 531 tube sets were 26.8. I can't believe anyone would use low-end tubing on a frame with such carefully shaped lugs. I am leaning heavily toward believing it is a pre-Fat City Chris Chance. I have yet to take off the fork to see what the steering tube has to tell me. The fork crown is unusual- it looks like an old-fashioned track fork crown made of stacked horizontal plates. I'll get a photo up later.