Not sure how good those old friction dampers are? The one on my LM2 had absolutely no damping force whatsoever no matter how tight I adjusted it so, whilst it looked the part it was doing nothing.
I had a an old Ducati 916 hydraulic steering damper lying around doing nothing so fitted it to the Guzzi.

I have since experienced quite a severe 'tank-slapper' though whilst riding over diagonal road markings so not sure how effective that is either.
Maybe it needs something adjustable to increase the damping effect as these Ducati parts are a fixed damper and probably not strong enough for the way it is frame to fork mounted which is different to how it was intended to fit (across the top of the fork clamp) on the Ducati, meaning it is using almost all of it's stroke compared to significantly less on the Ducati setup?