Moto Guzzi V700, V7 Special, Ambassador, 850 GT, 850 GT California, Eldorado, and 850 California Police models
Thanks to Charlie Mullendore of Antietam Classic Cycle for sending me his description and information via e-mail. In Charlie's own words:
Whittled this out in about 1⁄2 hour last night. All from scrap I had laying around, except the bolt and sealing washer.
The aluminum top has a recess cut into it for the 1⁄8 inch thick rubber gasket, the u-shaped piece clamps into the underside of the filler neck, the bolt has a sealing washer on it to prevent a leak there.
Hopefully once it's in place and clamped down, there should be no way for the MEK (that I'll be using inside to remove the failed POR-15) to find it's way out.