As Jerry said , Peugeot were probably Reynolds largest customer, certainly on the mainland of Europe. One of the Reynolds sales reps told me that the 501 tube set was specially developed, in the first place for Peugeot, so that the company could have a very price conscious but light and well performing "demi-course" bike.
That being said I dont think that the 531 sets were in any way special other than they were of metric sizing.
One other development that Reynolds did od for some part of tghe French market...I saw it on Mercier frames, was a 531 "seamed " tubing set. Again this was to reduce cost for the mass market. I think that something similar was used on some mid-range Raleigh bikes too, judging from some of the tubes I have pulled of damaged frames when repairing them.
Norris Lockley, Settle Uk