These pictures below show some plates from Aosta: they are exactly like the normal motorcycle plates, but have got also a shield inside as you can see. If you want to know more about the coat of arms, look the page about car plates from Aosta that have exactly the same shield.
About the numbering format, there's a strange thing that happened: the origin code AO follows the number, even if it is made by 5 or 6 digits (second picture), while it precedes the number in all the other Italian provinces: it was moved before the number, like all the other provinces, just in 1967 (third picture).
The last picture shows a plate with the coat of arms in inverted colours: I don't know yet exactly the time when these new colours were used.
Many thanks to Giancarlo Raposo for the first two pictures.