I s a cud possible without affecting the rim? Honest question. This does not seem to be affecting the rim at all. Pic will enlarge.
So, would this be a precursor to a cud that would eventually take in the rim at a later stage or is it a cud on its own just being at the edge like it is?
At the moment it is a rim to rim die crack which some call pre-cud. Once it deteriorates more and the piece falls away it will leave a raised area from the rim spreading inwards which we all recognise as a cud. IMO
To me it looks like the surface of the coin was peeled back before the copper coating was applied. I would be hesitant to call this a cud as the rim is completely intact and undamaged.
Im not seeing any cracks on the rim and feel its either glue or melted plastic. There are organic materials that are not affected by acetone. A solvent like Xylene will remove them (and like acetone, xylene won't react with the metal surfaces). If the OP has access to xylene and is comfortable working with it (use the same safety precautions as working with a highly flammable and reactive solvent like acetone) give it a soak and see if it can be removed. You could also check cuds on coins to see if they have anything that matches (sorry but I'm being and too lazy to check myself lol)