Yeah it’s the STC that Aircraft Door Seals has on their site. I bought the STC for mine and it helped but at annual we replaced all the bushings and bolts on the linkages up there too. All is good now, no more shimmy.
I'm new to the site and I saw your post and pic of Fox Island and I'm from Michigan. I joined the military and haven't been back much since but looking forward to airport camping in retirement. Is the strip smooth enough for an Arrow III?