I think I'll give these a miss. It hardly ever snows here sufficiently badly to warrant chains anyway. I'm going to give them to a friend who has a 1981 500SL in Zurich: they'll fit his tyres and his need is undoubtedly greater than mine.
John, part number is B 6 758 0258.
Thanks for your help, everyone.
1999 SL 500
1993 320 TE
1986 500 SEC