Games involving NHL teams and minor league teams or other entities date to the pre-expansion era. I recall publicity for a game played at Quebec (home to the Flyers' AHL squad) involving the Flyers and Canadiens and their AHL farm teams (Quebec Aces and Montreal Voyaguers) in what was billed as the first scheduled doubleheader in NHL history. That was in September 1969.
The first that I can recall was in September, 1955. Winnipeg Warriors of the WHL vs New York Rangers. Pre-Season at the Olympic Rink in Winnipeg before the Winnipeg Arena opened a couple of weeks later. I worked the Warriors vs Detroit Red Wings in Sept. 1959.