New Moto Guzzi Door Mats Available Now
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There is a metal loop that locks down where the panniers engage the bit so they cannot slide backwards once seated, but it's not on the rubber stop. The metal loop is forward of the panniers...is that what you are looking for?
I'm not sure that I understand correctly, English is not my first language. But if you're talking about the adjustable "thingies" sticking out at the licence plate with the rubber tip touching the rear backside of the panniers: Breva had them, Norge never did. Rode Breva for 6 years with and Norge for 3 without them. Asked my dealer about it and he said: no problem, Norge panniers won't fall off. They didn't, even though I suspect they were seriously overloaded from time to time. So, no sweat. If I misunderstood you, apologies.
The original rubber strap that held them tight against the spacer has never been available from MG.