This complex seems to be upgradeable (if you ever find that stuff.) Here we can find:

• IC20: P401A, matches the system ROM version (P401A60905A), so it's probably the main firmware;
• IC21: P402A, Google returns nothing useful, unknown ROM chip;
• IC24: CXK581000AP-10LL, or 131072-word × 8-bit High Speed CMOS Static RAM, which seems to provide our system with healthy 128 KB;
• IC25: idem, bringing the total amount of RAM in this module to 256 KB;
• IC26: OKI M6242B, an RTC clock chip;
• X2: 32.768 kHz crystal oscillator for the RTC chip;
• BAT: CR2354 battery, 3V, probaly of the /1GUF variant.

• The RTC battery is probably long dead and showing signs of oxidation on it's legs;
• There's was a bad and mysterious corrosion under the RTC oscillator, that I've already removed, after taking this photo;
• This same oscillator isn't operating reliably, sometimes it takes a while for the clock to start, well, clocking, and this component is probably the culprit;