Released in June 1978, Sea Wolf II is the first-ever sequel! It was basically the exact same game as its predecessor with a few cosmetic improvements.
Still, at least one of the cosmetic tweaks in "Sea Wolf
"'s case was a ground-breaker in its own right - though there's some argument over the fact, Sea Wolf II is generally thought to have been the first coin-op to use genuine colour, but was definitely not the first! Real color had been used as early as 1974 (in "Pace Car Pro
") and possibly in 1973 (in "Wimbledon
" and the "Color Gotcha"). These games, however, made very limited use of color (as did others, like Atari's "Sprint 8
") and most of them didn't sell very well. Sea Wolf II, on the other hand, sold fairly well and may have been the first game to make extensive use of color. Sea Wolf II probably represents the very first 'modern' video game.
About 4,000 units were produced.