I'm not actually sureâ€¦
If it was between one drive model and another, the comparison might include the few locations which are checked to verify the correct firmware is installed for a given drive ID. (People have managed to figure this out, which is amazing to me).
If it's between any two drives that have been used to burn, the comparison could include info stored as a part of the drive's learning mechanism.
And of course little bits of calibration data exist.
But comparing a clean EEPROM from a first generation drive and a clean EEPROM from a second generation of the same make & model (where applicable) might eventually show little flags that get changed upon upgrade. Specifically upgrading from 1st gen to 2nd gen, since going in the other direction has apparently proven difficult for unknown reasons. A full drive dump might be required in that case, though.