This is the basic schematic of the cartridge. Some GPIO pins from ESP12E module controls the output enable of the input isolators (541) as well as the output enables of the shift registers (595). It also generates the write pulse to the SRAM. The shift registers receive serially each address and data to be recorded on the SRAM. The SCK and RCK pins are connected because there is no need to synchronize the changing of the bits as the /WE pulse will go down at the end of the transfer when the addresses and data are stable. It also saves one GPIO pin.
Following thediscussion about voltage tolerance on the ESP8266 pins I have built a setup for testing the input characteristics using a voltage ramp generated by and arduino in series with a 1.5V AAA battery to ramp up the voltage to 6.5Volts and applied such voltage into the GPIO0 of an ESP01 module in series with a 100 Ohm resistor but haven't noticed the snapback on the input voltage as mentioned in the datasheet.
Later I have found that [ba0sh1] have performed a similar experiment but monitored the current consumption on the input pins. He haven't also noticed the action of the snapback circuit but both our conclusions are the same: The IO pins of the ESP8266 are 5V tolerant.
Next step was to experiment with the board and I have lost a lot of time trying to figure out why to the module got stuck so often until I realize that the parts list of the modules are really kept to a minimum and even the reset line lacks a pull-up resistor! So it is very important to pull up all of the pins (at least with the original firmware).
For doing that I have built a breakout board based on this web page. Now I can move on and load the micropython firmware to experiment with it.