Entex Hockey, head to head style or against the computer. It was originally released in 1979, and didn't have any difficulty settings. When it was re-released in 1980, an Amateur/Pro switch was added to the opposite side of the game (from the on/off switch). The box artwork was changed to reflect this, and the U.S. box was updated to a completely different style.
Entex was one of the leaders of the 2nd wave of Handheld Electronic games. Their games tended to be more complex than those put out by the competition but still simple enough to be easily picked up and played without much of a learning curve.