Molgrips, not sure if this helps or not, but I bought a pair of 55 ATA2 forks 2nd hand and the travel was stuck down due to air escaping into the -ve chamber of the ATA cartridge, which was a common problem with the 2009 range due to dodgy o-rings according to a forum post at MRB.com
Anyway, having got the required o-ring size (or one that was nearest it) I took a punt at the following item on eBay:
http://www.ebay.co.uk/itm/116-IMPERIAL-O-RING-10-PACK-SIZE-18-72MM-ID-X-2-62W-/290594541043?hash=item43a8c8cdf3
This size fitted the main air piston in the ATA cartridge really well, and 12 months on, still works perfectly, no loss of air and the travel stays up at 160mm. So it basically cost me 15p to fix, and to answer your question, I can only assume that the ’70 Shore Nitrile’ material these claim to be made from is good enough, and clearly better than what Marzocchi were fitting themselves 5 years ago 😉