In the early nineties ducati took what they had on the laying around as Antti says
so it´s quite normal to see -91 907:s with both black and gold brembos but as
Francesco says -91 , 300 mm diameter discs, - 92 320 mm. The silencers are also
different (most of the time) although you can see - 91:s both with black silencers
and the newer type with aluminium wrapping. The seat is also a little bit different
in seat height, I have both types of seats and the newer type is lower. My - 91
also has aluminium wrapped silencers, but black brembos. I believe the - 92 type also
have a lockable gas cap??

well, that would help make the case for mine being a 91.
i do not have a locking gas cap.
interesting point, i didnt know any of the pasos had these.

Judging the year by the type of gas cap is risky at best. I'm pretty sure the later cap will fit an earlier tank with no problems so it doesn't prove anything either way.

Its very easy to loose key and change newer type of gas cap to older modell and also via versa. That is quite small thing compared to saddle/mufflers/rotors/calipers/ECU(P7/P8).

and beautifully maintained it was.
still get lots of people ogling it unwilling to believe that it is now 16 years old !