Diamond should have a different MCU ROM than the one in Sapphire.

I did look for an MCU pin that might be GND or VCC depending on hardware config, but did not find it. And the thing that convinced me was the serial interface ID. The "SAPPHIRE" string is in the internal ROM, and no sign of "DIAMOND". For Diamond II/Sapphire II, it has both strings in EPROM.

Before anyone asks: I don't think Star Sapphire has the same ROM as Star Diamond.