Jim, do you know what ages your Galaxy and Super Glaxy were?
The Super Galaxy was a 1990 model - Post Office red with full LX. The Galaxy had been resprayed, but was mid nineties. I also built a late eighties Galaxy for a friend about ten years ago, and remember very clearly the 27.0mm Ritchey seatpost that went in snugly.
Have a look at this chart:
http://www.equusbicycle.com/bike/reynol ... -sizes.jpg
The standard 531ST set has a 0.2mm thicker seat-tube wall than 531c. That implies a 0.4mm smaller seatpost than a typical 531c frame. But again, there's often more variety to be found in real bikes than in the catalogues (or in the selection of catalogues that have found their way onto the web), and the only way to be sure is to measure, or try a few different posts. Your local bike shop might have a sizing gauge if you can't lay your hands on a Vernier caliper.
Calmes dans le demi-jour
Que les branches hautes font,
Pénétrons bien notre amour
De ce silence profond.