Mizzen Staysail
We use ours regularly, usually with the spinnaker up. It make a difference and is pretty neat. It blankets the main when going dead downwind, and doesn't add anything. It works well on a reach by itself as well. The tack is normally at a padeye on the cabintop just outside the handrail, but we have moved it out to the aft shroud turnbuckle when we were trying anything in a race - I think it might have made a difference, but who really knows. I have put an adjustable pennant on the tack to allow adjustment.
The staysail and spinnaker may be original sails - and hence pretty old! I would love to see the mizzen spinnaker. I also understand that some Challengers may have had a spinnaker staysail, but I have never seen one.
Rick