Brucey wrote: 531 was always available in a wide variety of different gauges...
The actual metal from which the tubes were drawn may have been, but is that true of tube sets specifically designated as "531C" and "531ST"? Apart from a small number of custom builds, weren't their specifications reasonably standardized?
when both tubesets were launched, there were few if any 'options'. The rationale was that in more mass-produced frames, the customer would know what they were getting, in contrast the previous (apparently confusing) situation where a touring bike and a racing bike might have the same sticker on the frame but different gauge tubes within it.
Of course it is an utter nonsense to use the same gauge tubes for different sizes of frameset, even if they are intended to be used for the same purpose, so various options were introduced. If you bought a handbuilt frame (even with a particular sticker on it) it would quite likely have tubes that differed from the standard specification, if the builder deemed it to be of benefit.
Small changes in frame performance could be achieved by trimming the tube butts differently etc. However generally speaking the standard tubesets might have built into reasonable frames for riders of average weight (70-80kg) and frame size (22-23-1/2" typically) but outside that they were decreasingly useful.
BTW if you ever have the opportunity to ride the same tubeset built into differently sized frames, or even just very differently sized frames, take it. A very small frame built in the lightest possible tubes can feel stiffer than a very large frame built in the heaviest possible 531ST (standard diameter) tube gauges.