Technically, the transaction had two steps: i) conversion of ZTS Class-B (super-voting rights) shares held by PFE into ZTS Class-A shares; and ii) “disposal” of 401M ZTS Class-A shares in the exchange for PFE's own shares.
ZTS now has only one class of shares and PFE has no equity stake whatsoever.
“The efficient-market hypothesis may be the foremost piece of B.S. ever promulgated in any area of human knowledge!”