The Snake Grips seen on Eastwood's gun in the Dollars Trilogy were hand-me-downs from Clint Eastwood's earlier TV Series Rawhide.
He acquires them in episode 2 of season 1 of the show (Titled "Incident at Alabaster Plain") from fallen bad guy Ward Mastic.
At the end of the episode Eastwood's character Rowdy Yates shows off the grips to a coworker and comments:
"Solid silver inlaid in ebony. Must have cost a lot of money."
"It's a rattle snake. Howdya like that?"
"I've seen men engrave their initials in gun stocks but never anything like that."
The snake grips' origins are said to have been from leather maker Andy Anderson's shop. It seems that other shows Anderson was involved with also had the snake grips. Notably The Wild Wild West (TV Series 1965–1969) where the show's lead Robert Conrad who played James West was seen with a set of the snake grips.
The sterling silver rattlesnakes, coiled and ready to strike, were supplied on the custom grips by Andy Anderson of the North Hollywood Gunfighter shop. It is unknown who crafted the original snakes, but they were definitely professionally made by a silversmith.
I was told that Andy Anderson carried [The Rattlesnake grips] in his shop back in the day.
It should be noted that Andy Anderson was a leather maker. Not a sculpter or a jeweler and his tool set almost certainly wouldn't have included the equipment for mold making and casting metals. It's nearly certain that he sub-contracted the job of creating the snakes out to some local jeweler. Although many people have tried through many attempts over the years no-one has ever been able to identify the original sculpter that created the snakes for Andy Anderson.
In this site's editor's opinion, the fore-most expert on the snakes is Joe Perkins of Classic Single Action in Arizona who has put years of work into researching them and has created the closest copies that exist.