Perhaps this is cheating, but I tend to be convinced that a Mac is New World when the Document Mac OS ROM appears in the System Folder of OS 9 ... and, conversely, that a Mac is Old World architecture when, having got a Mac to take OS 9.2.x on board, I find OldWorld Support 1.0.1 in the startup splash screen ...

But, Siriusly, I think that Drew has hit upon a nice overt sign of the difference.

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(60x) 13DT + 3PB + PTPro; (G3) 7DT/MT; (G4) 3T + PB. System 8.1 to OS 10.5.8