This thin piece of rubber/foam?https://www.amesperf.com/parts/4287/
You mean this one inside lip of shaker adaptor ring? #527230. It changed profile a few times, being solid or hollow, ended up crushed flat and hard, no grip and shaker turned. No one 'really' sells it, although they often market a generic rubber seal that may fit and do the job. Nothing special, adhesive-backed and keeps shape, but just thin enough that the clamp can slip over it to begin with. I'm not sure what the Ames seal is meant to be, or the application years which don't make sense. So maybe there's something else, like regular air cleaner lid seal.https://www.firebirdnation.com/threads/shaker-scoop-gasket.568991/shaker3 seal1 by Ben, on FlickrGM NOS 527230 shaker base adapter to aircleaner seal TA (2) by Ben, on Flickr