Bank Alarm??

When the gang first gets off the train in Bolivia, as Sundance is walking up to the entrance-way, in the background, you'll see a MOTORCYCLE fly by!

Must a been....just went back, and the scene where "Sweet Face" is supposed to throw the posse off & Butch and Sundance are in the prostitutes room, an ELECTRIC LAMP is clearly plugged into the wall...

Western Electric Company patented an automatic, electric fire alarm system in 1871...for use in large buildings like hotels called an electro-mercurial annunciator. I'm pretty sure banks would have some sort of electrical alarm system in the late 1800s.

Although I don't think the film actually dates any of the events in the third act, it does feel like it compresses time.

The US events were turn of the century, and Bolivia was some eight years later, although the film doesn't seem to give much indication of that.

But in either case, electricity was hardly unknown by then. I think the subtle nods too it were entirely deliberate.

I lived in an 1890 house, and it was originally built with a combination of electric and gas lighting. Only three or four outlets in the entire 1,500 square foot place--they didn't have much to plug in back in those days. We had long extension cords running everywhere and blew a lot of fuses. Anyway, the point is electric power had been around for a while at the time the movie was set.