DE is the years 03-06 (276 BHP)
RevUp is years 06-07 (297 BHP)
HR is years 07+(313 BHP)
MY is just the year, so I had a MY07 (297 BHP) car
No you didn't.
You had a MY06 on an 07 plate.
MY03 - 276bhp (03-55 plates)
MY06 - 297bhp (06-56 plates)
MY07 - 309 bhp (07+ plates)
Naturally some cars got registered late, hence your car was registered as an 07 plate but was actually a MY06 car.
Well, you learn something new every day