Complex sentence: Rumors that the Soviet Union had produced a swept-wing fighter had been circulating since 1948 and Mikoyan-Gurevich MiG-15 was widely seen at air shows in 1949.
Split into simpler sentences: Rumors that the Soviet Union had produced a swept-wing fighter had been circulating since 1948. The Mikoyan-Gurevich MiG-15 was widely seen at air shows in 1949.