The coca leaf, which is and has been widely used for medicinal and restorative purposes in Bolivia and surrounding regions, and is featured in at least one scene in the film, is commonly said to resemble the leaves of a blackthorn bush.
The song that Blackthorn sings when the two of them finally get a horse is the same song that Clint Eastwood sings in Two Mules for Sister Sara (1970) when he is drunk.
Prologue: "Butch Cassidy was one [of] the most wanted outlaws in North America in the early 20th century, leading legendary gangs like the Wild Bunch and the Train Robbers Syndicate. Hounded by the law and big companies, he fled to South America with his friend the Sundance Kid. Both were supposedly killed in a shootout with the Bolivian army in San Vicente, Bolivia, in 1908. Recently investigators analyzed remains found at the location where witnesses had claimed they were buried. They were not there."