Joystick interface for C-16, C-116 and Plus/4 - V1.1
Reading C64 Joysticks and Analog Signals (Programming Electronics with the Commodore 64 Part 3) - Retro Game Coders
C-64 Workshop / Pin Connections
Technical corner
Joystick – C64-Wiki
Tube Time on Twitter: "the left one is a Commodore 1351 mouse designed for a C64/C128, while the right one is meant for an Amiga. the pinouts are very close and neither