I guess that will be an SSS mast, similar to that shown for a Reedcraft boat at:
http://www.seahawk17.org.uk/description/rigging.htm#mastUnfortunately, SSS went out of business many years ago, so you're probably best looking at boat jumbles and the like. I don't have the experience with other boats to know if the casting is a common one used on the mast of any other boat. Hopefully, others will have more helpful ideas.