The Q2 spec cars were 1.9JTDM only, but that is not to say that your V6 won't have a Q2 diff, it could have easily been spec'd and could have been fitted as after market accessory, as the Q2 diff is compatible with gearboxes fitted to the JTDM and V6 (not sure what 'box the 4pot petrols had). Even if it wasn't offered as an official spec option it will have been something a dealer could easily have offered.
As I said, whilst Q2 is known as the branding/part name for the limited slip diff, it was a spec of vehicle as well, so it should be detailed on the V5 and probably have the nice red and silver Q2 badge on the back.
I think official Q2 production stats are showing production between March 07 and Jun 09. But just because you don't have an official Q2 spec car, doesn't mean you don't have a Q2.
Nice and confusing for a sat morning.
1999 156 1.8TS Sport Pack 3
2001 166 3.0 V6 24V Super
2003 156 GTA 3.2 V6 24V
2005 156 1.9 JTDM-Jet Sport
2008 159 1.9 JTDM Lusso
1997 GTV 2.0TS Lusso
2001 147 1.6TS Lusso