M42 adapters

Is there more than one type of M42 adapter? I recently bought a 500mm mirror lens from Ebay that had a screw-on adapter for a Canon. As it measured 42mm across I assumed that it was an M42 type lens and bought a PK-M42 adapter for it. The thread (pitch?)on the adapter that I bought is coarser than that on the Canon-M42 adapter and will only go one turn before it seizes and won't turn any further..

That sounds like a T2 mount to me:

There's more info here

You can get a T2 to PK adapter that screws on your lens and has a K mount bayonet that fits straight on to your camera.

That sounds like the answer. Just wikid - the pitch on the T2 is 0.75mm whereas the M42 is 1mm. Thanks very much! Off to Ebay again