Hope this helps...
DEPARTURE ARRIVAL FLIGHT FREQUENCY
FCO 15:15 LHR 17:00 AZ206 M T W T F S
The flight is from FCO-LHR and seems to operate on Saturday's only. Assuming, from the AZ timetable I have in hand, this flight is operated by an:
So, while definitely not a 763 or 777, there is an AZ 206.
Also, it could possibly be an A/C substitute or, perhaps if you saw a 777, it's their third 777 (I-DISO "Positano") doing inter-european crew training flights.
Hope this clears up some of your confusion!
"Alitalia. Dedicata a te'."
Some see lines, others see between the lines.