Complex sentence: He reaimed sitting in for Charles from 1974 until Match Game episode 471 when Charles returned in 1975 from Broadway, even though at the time (74-75) he was playing his character Radar O'Reilly on "Mash".
Split into simpler sentences: He reaimed sitting in for Charles from 1974 until Match Game episode 471 when Charles returned in 1975 from Broadway. And while at the time (74-75) he was playing his character Radar O'Reilly on "Mash".