I don't know if this has been discussed, in case I'll obliterate the topic. But when Bronson first appears he's not playing the harmonica which has made the 3 turn back because he's got a Gladstone in his right hand and something else in the left (his gun belt?) which certainly is not a harmonica and, even more certainly, he's not playing it.
https://www.youtube.com/watch?v=ZmxZN91I70kMy guess is that Leone had devised another entrance but had second thoughts a few hours after having shot the scene and, not having maybe the train available (or because of Mulock's suicide) had to leave the scene of Bronson's entrance like that. But it would be interesting to know what it was that made the three turn back before he thought of using the harmonica.