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RAM types 
The serial chip is a NEC uPD42832C. This is a 32K pseudo-static RAM chip. These chips are like dynamic RAM but have the ability to retain data under a standby voltage (around 2V) with a self refresh circuitry.Dynamic RAM chip are incompatible. Static RAM chip can be use without any problem. The replacement is recommended because theyre their power drain is very the lowest (1/10 ratio).

RAM socket 
The motherboard layout has 32 pins. It is tracked for a 128K chip. On issue 4 machine a 32K chip is soldier using the 28 low pins. Here is the socket pinout from the component side view.