Super Spy Library #1

Page Publications' Super Spy Library #1 was published c.1974 and features a rather moody Keith Chatto cover:

The cover is swiped from MV Features' Secret Service Picture Library #24:

The protagonist of The Face of a Traitor, Gerald Blair, was a recurring character in the Secret Service Picture Library series, and thus appeared in a few Page Publications issues, including the many printings of The Edge of Fear. 

This is the only Yaffa printing of The Face of the Traitor I have found so far, but I daresay more will turn up.