Here's the pin assignment for the PCB:
1 - Clock
2 - Ground
3 - Data
4 - VCC (?)
5 - LED (When this is LOW and 4 is HIGH, the LED is on.)

I'd like to point out that the LED pin might have some other connections since it seems to be pulled high/low without an electrical connection to the printer. However, I haven't been able to confirm this.