Official documents show that Una Colt, 5 dollari una carogna and Sette Colt per sette carogne are the spaghettiesque versions of El Mexicano - El Retador and La vuelta del Mexicano respectively, both Mexican productions directed by René Cardona Sr.
Practically all actors have only 1 credit because the names - just like in Adios Cjamango, the Italian version of Los rebeldes de Arizona by Zabalza - are pseudonyms. William Cliff should be Jorge Rivero.
Both films are available on YouTube in original version. In the spaghettiesque versions SW music was certainly added, and since the runtimes for the Italian versions were 84 and 82 minutes a few additional scenes for 7 Colt per 7 carogne (see Reply#13) seem confirmed.
If you are not convinced, compare this screenshot from the YouTube version with the lobby card...