Gas cap replacement
Moto Guzzi V700, V7 Special, Ambassador, 850 GT, 850 GT California, Eldorado, and 850 California Police models
Civilian flip top cap
I currently know of no suitable substitutes for the original cap.
Police screw type cap
Drag Specialties gas cap
I had the police style cap on my Ambassador (just a round cap, no lever). Although my gasket was good, I kept getting leaks through the venting mechanism. I kept hoping to be able to get a NOS or good used one off of ebay, but no luck.
John Ulrich told me that he replaced his with one from Dennis Kirk. Finally sick and tired of cleaning up leaked fuel, I purchased the vented cap from Dennis Kirk (Dennis Kirk part number: H18199; Drag Specialties part number: DS-390130).
It looks very close to original, fits well, and seals perfectly. When changing out my petcocks with a tank nearly full of fuel, I set the tank on end for over 10 minutes without any leaks from the cap.
Gregory Bender's thoughts:
After using the cap for a while, I've discovered that it let's air in fine, but does not allow air to escape. This may not seem like a problem, but too much pressure inside the tank will stop the fuel flow (I'm not sure how this happens, but it does). This usually isn't a problem unless you are running long distances at speed. Pressure will build up inside the tank to some point, and then the engine will falter. Open the cap to allow the excess pressure to escape, and the fuel flow continues and away you go. This seems to happen more to me on hot days than on cool days. John Ulrich drilled a couple of strategically placed holes on the underside of his cap to prevent this from happening. I did the same, but then the cap started leaking again (I like to fill my tank up very full). So, I'm back to the drawing board. Until I find a better solution, I will continue to use the cap from Dennis Kirk.