I was beginning to wonder if I'd misremembered, but Wikipedia's list of Ace Doubles shows: "16641 SF Jack Vance The Dragon Masters / The Last Castle (April 1973)". So, I even got the time right.
I'll have to root through Dad's attic, to see if it turns up. So much has wandered away, over the years.
44 posted on 02/02/2013 7:42:56 AM PST by Lee N. Field
("You keep using that verse, but I do not think it means what you think it means." --I. Montoya)