Here's the cover of the first Executioner book, published in 1969, featuring action hero Mack Bolan. An interesting pop culture fact: In the 1970s, Marvel Comics based their anti-hero The Punisher on Don Pendleton's Bolan character. Both were Vietnam War veterans who had their families murdered by the mafia and were driven to seek revenge. Additionally, the "War Journal", battle van and weaponry were all used in The Executioner series before the Punisher was created. Thanks, Wikipedia!
Check out the mackbolan.com fansite for almost 40 years of collected Executioner book covers.