Does it matter? It's like asking which Batman film was first. I think Leone just got a good script and took the same character. So Clint Eastwood plays the same guy, only not. It's like the Mariachi in
El Mariachi and the one in
Desperado. By the way, there is going to be a third film in this trilogy, called
Once Upon a Time in Mexico.
Also notice that actor Mario Brega plays different characters in each film.
So I think Leone meant nothing by it, but it would be nice to consider GBU the first film...
Or the poncho could also be something like in Raiders of the Lost Ark, when Indy, opposed to the swordman, pulls his gun and shoots him. There was no indication in the film that he even had one. But Spielberg let it in, saying that no one would notice it. And I think it's also the case here...