The original link is just the datasheet for the PIC, and doesn't explain anything about the
board. That is a very old style PIC, which has no on-chip peripherals whatsoever, such
as I2C or RS232. The "C" version is before-flash, so the chip could be programmed one time
only, so you're stuck with the existing code, short of unsoldering the chip from the board.
However, the I/F has 4 wires, no doubt 2 being power/gnd and 2 being control/comms.
I would look at the control/comms lines on a scope and try to figure something out. First
check if the lines are input or output. Maybe data comes out constantly. Proceed from there.