Well I'd be happy to say that the frame is all 753. They might have substituted lower range stays, but they would still have to have used a lowish temperature rod for the seat cluster fillets, because of the heat treated 753 main tubes, so wouldn't have gained much. The 653 tube set used 753 material stays, too.
As for the braze material colours, they'd have probably used high silver content alloy ( eg: SIF43 55%Ag, 630-660ºC) for all the lugged joints. The tight working temperature (solidus/liquidus range) gives it better capillary action, quick to build (lower heat volume) with the low temp characteristics needed for 753.
However, the stays and seat clamp bolt required a large volume fillet, so they would have probably switched to a lower silver (eg: SIF39, 38%Ag, 650-725ºC) thus cheaper and more bronze coloured alloy for those. The wider solidus/liquidus temp range allows easier fillet building, with less need for very tight temperature control, but still keeps temps fairly low. It is also less prone to cooling cracks in fillets than the high silver stuff.
Straight bronze alloys used on most cro-mo and mang-mo frames runs much higher (eg: tight range SIFBronze #1, 870-890º for lugs and wider range SIFBronze #2, 920-980ºC for fillets).
The 700ºC required to build a fillet using something like SIF39 would still be quite safe, as the joint is onto the back of the thick lug, away from the main tubes. The stays themselves were nominally 0.6mm wall (but generally thicker because of the swaging required for the double taper) and unlikely to fail up at the top, where the triangulation is so good. Personally I usually silver brazed a plug fitting top eye into the stay, then fillet brazed the top eye onto the lug, but this is more expensive than the direct approach here.
The previous silver brazing of the seat lug onto the seat and top tubes would not have reached a temp high enough to affect the main tube joints as it requires more heat to un-braze than braze freshly. Also the larger mass (compared to the seat stays) of the top tube and seat tube would have kept the main frame temperatures low while brazing the seat clamp and stays.
The braze-ons would have been 40-45% silver again for speed (lower costs) and lower heating.
Also it's really easy to overheat tiny parts like those top tube cable guides if trying to get to brazing temps (800-900ºC) at the same rate as the tube to which they are being fitted. The guide can be red hot before there's any real sign of the tube or flux warming. Much better, easier and quicker to warm the area of tube around the braze-on and let conduction heat the small part to silver temps (600-650ºC).
All the best,
