Whiteboard
Whiteboards became commercially available in the early 1960s, but did not become widely used until 30 years later. Early whiteboards needed to be wiped with a damp cloth and markers had a tendency to leave marks behind, even after erasing the board. Dry-erase markers for whiteboards were invented in 1975.
Chicago Stadium
That clock eventually was replaced by a four-sided scoreboard with a digital clock, first used on September 21, 1975 in Blackhawks preseason play, crafted by the Day Sign Company of Toronto, much like the one used at the end of the 1960s (and constructed by Day Sign Company) to replace the nearly identical Bulova Sports Timer game-timekeeping device in the Boston Garden, and then in 1985 by another, this one with a color electronic message board. That latter scoreboard was built by White Way Sign, which would build scoreboards for the United Center.