Atari released its ST home computers. This mouse probably was designed for that.
ATARI STM1 ATARI CORP MADE IN TAIWAN SERIAL NUMBER A1 5010799
According to Wikipedia, "The Amiga and the Atari ST use an Atari standard DE-9 connector for mice, the same connector that is used for joysticks on the same computers and numerous 8-bit systems, such as the Commodore 64 and the Atari 2600. However, the signals used for mice are different from those used for joysticks. As a result, plugging a mouse into a joystick port causes the "joystick" to continuously move in some direction, even if the mouse stays still, whereas plugging a joystick into a mouse port causes the "mouse" to only be able to move a single pixel in each direction."
Presumably the Atari mouse knows how to behave on a joystick port.