It has to be the ten tooth SRP cog on my rear cassette, replaces the lockring on the block, changes 8 speed to 9(except with eight speed spacing so no modern 9 speed shifters work - only my mavic wishbones which are 8 speed spaced but have 16 or so clicks!), fowls the rear drop out, is utterly pointless unless you have the legs of the man Ullrich, but is totally cool as i can't think I've ever seen another (which is probably for good reason, like it was a daft idea in the first place)