I am seeking a 1960-70s pulp-sf novel. I read it in the early to mid-1970s and it was older then, so guessing it is from 1950s-1960s. It was about a Galactic agent/military officer with an artificial hand who was sent on an assignment to determine the motives of a wealthy tycoon (possibly an alien species, but humanoid). In addition to the usual bad guy thugs, the wealthy tycoon had an attractive looking female assistant as the love interest. Looking back, the plot reminds me of a James Bond type adventure.
As I recall, the cover was green with a sketch picture of the hero from the shoulders up, head tilted to the side. From what I remember now, it reminds me of what a British RAF officer might look like. Pencil mustache, aristocratic features, dark hair. Think, David Niven for example. The female protagonist might have been in the background over the right shoulder of the hero.
Three scenes stick out in my head:
During the introduction meal with the tycoon, they watch an "entertainment" in which players move on a defined grid pattern on the floor and occasionally kill each other. It reminds the hero of "human chess". I think I remember the tycoon admits people from his planet had been to Earth millennia before and might have influenced human development.
At some point, the protagonist and the female assistant are locked into an sea grotto where the tide is rising. As it does, it starts to fill with sea-crustaceans (think killer crabs). The hero is able to use his artificial hand to individually crush them, thus saving the day and allowing them to escape.
I believe there is a scene in which the tycoon is describing the speed of a warship and uses the term "it is doing 20,000 lirgs". The hero approximates that to miles and says "Oh, it is only cruising then."
It is not Chessmen of Mars by Edgar Rice Burroughs. Also, it is a true artificial hand, not a "Gil the Arm" like adventure with his phantom esper sense and telekinesis.