Was that a 500cc P twin? Not many of them around anymore.
Yes, parallel twin 500 GTL.
Bought it brand new from the showroom in Alexandria Virginia in 1977 for $1800 USD cash, and the dealer included a Bell Star helmet and some Napoleon mirrors. Orange with silver stripe. I wanted a 750 at the time, but the 750 was $2400, and I didn't have enough cash at the time. So, I bought the 500.
It was a great bike. Put a huge number of miles on it. Extremely reliable, reasonably fast, could do The Ton, handled great. I loved it.
The only issue it had was burning up the condensors which were located in the distributor housing. Once I re-located the condensors on to small tabs near the ignition coils where they could get some cooling, no problems after that.
I know that model is the "Odd Duck" and gets little respect among the Ducatisti, but most of them never owned one. I have also owned two of the round case 750 twins, and although the 500 is not in that classic vein, it is a very nice bike in its own right. Parts are now very hard to get, so it might not be a great choice for a daily rider today.
It looked exactly like this one: