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I applaud the attention to seat post sizes, but I have some bad news for you guys.
My Norman three speed from the early 50's, which is not 531, takes a 26.4 post. I read somewhere on Sheldon's site that most Pre-Raleigh or Non-Raleigh three speeds took a 26.4 post, while Raleighs take a 25.4. He does not say so, but this is true of all Raleighs of that period, including the 531 ones; my 1951 Lenton Sports (straight gauge 531) my departed 1958 Lenton Grand Prix (straight gauge 531), and my 1948 Raleigh Record Ace (butted 531) all take 25.4 posts. I also have a Falcon from the 60's that takes a 26.2 post and it has a 531 decal, the type that indicates straight gauge tubing. For completeness' sake I'll also mention that my 40's Fothergill (unknown tubing, but it looks like it came with a 531 decal) takes a 27.0 post and my (departed) 1950 Norman Rapide (butted 531) was 27.2.
If you completely unbuild the frame, weigh it carefully. This will tell you a lot. Weigh the fork separately. I don't have my notes with me now, but I can tell you my RRA is considerably lighter than all the others.
I would like to see photos of your Portland. For some reason I have a Portland head badge.
Haha! rhm, glad you tuned in! I will post pics in a few minutes so please stay tuned!
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You see, their morals, their code...it's a bad joke, dropped at the first sign of trouble. They're only as good as the world allows them to be. I'll show you. When the chips are down, these...These "civilized" people...they'll eat each other. See, I'm not a monster. I'm just ahead of the curve