As from the factory the 196 Super Sport had a Villiers Medium Weight (MW) carburettor. This clips on to a 1.125” machined mounting stub on the inlet manifold. The carb itself has several fine slits (usually four) in the clamping area allowing the clamp to squeeze it down on to the machined stub.
You have two issues here.
Firstly the manifold stub is worn out of round and under size. It is a machined surface and should contain no casting marks. I suspect those you describe to be wear and tear.
Secondly, the fine slits mentioned above will be closed up due to previous owners tightening the clamp more and more trying to take up the stub wear. You cannot clip the carb on firmly until they are returned to as made. It is no great problem to gently ease them back out to size again. I use a specially tuned solid steel stub for the job but the right size ball pein hammer should do just as well. Just remember slow and gentle.
When the carb is back to size you can start on the manifold. Perfection is a new manifold, been there done that but it’s a real pain no matter how you go about it. Secondly you can replace just the stub, turning up a new one to have it welded in place of the old. Also fraught with drama as welding new to old aluminium is a tricky business. Villiers did concede a sliver of shim brass as suggested by R as a temporary fix and basically that’s all you can do until you get a better one. Shim brass is available in many thicknesses so no drama .
You’ve already found out that plastics and such aren’t keen on prolonged contact with petrol. Brass doesn’t care that much
