I think what Shaun was eluding to was the link to the other bike show's the tubing as 18-23 Hi Tensile steel, perhaps inferring that the 501 sticker on yours isn't original? The person who has contacted you through ebay obviously doesn't like the fact you've got it listed as 501 tubing? Perhaps modify your ebay advert?
Must admit google doesn't provide many answers but if this is just so that your ebay ad isn't seen as misleading then better to be safe than sorry.
Rick Powell 753 Track / Ribble Gran Fondo / Colnago Asso / 1975 Harry Quinn TT / 1990 Concorde Prelude / Vitus 992 / Keith Coppell Lo-Pro / 1986 KAS Vitus 979 / 1995 Mercian Strada Speciale / 2011 Raleigh GAC Militis 3 ex Graham Briggs / 1995 Ribble 7005 / 1986 Alan Super Record / 1985 MK1 Vitus 979 /
On the bench: Sirius 853 TT / Sun Solo Chrome / Ciocc Designer 84 / Giant TCR0 Pro Compact / Ribble 653 Tourer / Daccordi Lo-Pro / Look KG96 / Bottecchia Equipe / Look KG231 Credit Agricole / Contini Grand Prix / Aende Lo-Pro / TVT / Langdale 653 Pursuit /