"The rarest early Anser of all is the Anser X." [And The Putter Went...PING (2017) by Jeffery B. Ellis]
The putter heads were originally cast to be Ansers, but the hosels did not turn out right. So the clubs were made with the shafts straight into the Anser heads.
All Scottsdale Ansers are rare, as they were only produced for a few months. The idea of the Anser was first sketched in January 1966. By mid-1966, PING moved from Scottsdale to Phoenix, and all future clubs would bear the Phoenix name.