Well, I figure that the US is probably a pretty good bellwether for drugs laws about the world. At the same time, the earliest real drugs law (I don't think we can count alcohol) was in the 1870s, after the US Civil War (which is what I assume you were referring to), and, in fact, during the period in which we would place the actual Wild West. While I don't think that it's necessary that the veribiage used indicates that the event happened after the Wild West, it'd seem an unlikely choice of terms to label a contemporary event that way. Which probably places this in the 20th century, as it's likely that drugs laws didn't exist until the 1870s and the Wild West itself lasted pretty much to the turn of the century.