For the C1-platform cars (C30, S/V40, V50) used to use the McGard SL 'spinning collar'-type locking nuts that a lot of other manufacturers (Ford, for one) use.
They came with a card that had a code number on it - you can go onto McGard's website and buy a replacement key based on the code.
The locknut sets McGard supply to car manufacturers usually have a restricted range of keys so the dealers could order 'master' sets. The card that comes with a Ford set of McGard nuts has a single colour code, and you can order a replacement key from a Ford dealer based on the colour.
However, the McGard design has some well-known weaknesses if they are used incorrectly (being installed or removed with an impact driver instead of manually), or if you are unlucky enough for one of them to seize. The part of the key that fits into the nut can wear and deform, or shear off in the nut, and the head of the nut itself can shear off, leaving a small steel sleeve stuck on the stud.
It seems that around 2005/2006 Volvo decided to change their supplier of locking nuts used across the range to the
Heyner ChromeSafe Pro model that has a toothed outer ring and fake spinning nut in the centre:
This is theoretically a better design as the larger diameter mating surface and increased number of teeth should make it stronger and allow it to handle more torque in the event of seized nut (Heyner's specs claim a rating of 400Nm)
As has previously been stated, however, the dealers say they cannot supply spare keys. They are coded, however (if you look inside the socket there is a single letter stamped inside). The use of a single letter indicates that there can't be that many combinations available, presumably to make the 'master' sets supplied to dealers manageable. If I was being cynical I would suggest the policy of not supplying keys separately is a way to make more money by selling a complete locking nut set instead.
Strangely you do sometimes see keys of this type listed on ebay identified by the letter (there are a couple up on there now), presumably from cars being broken, or complete sets that may be worth buying for a spare key alone. It may also be possible to obrain a spare key directly from Heyner if you know the code letter, as the they can supply spare keys for sets purchased directly from them.
Interestingly if you buy a locking nut set direct from Heyner they are also half the price of a set in the Volvo packaging (£30 versus £60)
http://www.wheellockingnuts.co.uk/chrome-safe-wheel-locking-bolts-m12-x-1.5-type-i3/m
jactac · updated 
Barnsey · updated 
VoodooBOB · updated 
