Hard to be sure which alloy was really used. I picked on SIFbronze #1 because it is one of the original rod compositions that is still available. The company describes it as well suited for general purpose brazing and bronze welding of steel, cast iron, etc. I know SIFbronze was frequently mentioned back in the day as a preferred filler rod for bronze welding frames, both mild steel and 531 alloy, but I've never seen any mention of which alloy rod was used. More recently, John Bradley mentioned in his book on motorcycle design and construction, that a good rod for bronze welding 531 frames was SIFbronze 101, which also has a high percentage of zinc, being 60% copper, 0.2% manganese, 0.1% silicon, 0.1% tin, and the balance zinc. These references explicitly call out bronze welding, in addition to brazing, as a suitable application. More info here, including some history of SIFbronze.
http://www.weldability-sif.com/pages/home.asp
I've just gone through Ken Spraysons autobiography again, and although he gives a lot of detail about construction of featherbed frames, including that all Manx frames were bronze welded Reynolds 531 tube, he never mentions the type of filler rod used.
Fascinating stuff. Interestingly, the Weldability-SIF site recommends one of their phosphor bronze rods for brazing and bronze welding with tig. I'll have to try that, but that will have to be another thread.
Ken