The older versions (C and CT*) have, as opposed to the newer CF/CFi/CFE-lenses a threaded front, not a bayonet. This thread has a diameter of 63mm, which is a pretty unusual size today. Unfortunately, I don't know where to find an adapter, although I have seen similar adapters for the 50mm thread on *bay.

I use a B60-62mm adapter and then a 62mm Cokin holder, much cheaper than the Cokin B60! =)