I replaced the front end bodywork on my P220 when I got it home and I *think* I know where your rubber gasket goes. The gasket likely goes on the fender up at the car's "A pillar." My Amazon had a rubber seal gasket there on both sides. I've been waiting to finish my mechanical work before I got back into bodywork and reinstalled the seals.

Check it for length - it ought to be just right.

Also, there was a grey, clay-like sealant on my inner fenders under the attaching bolts. I found some similar material at a local Habitat For Humanity Store. I think you commonly find it used for HVAC or as a winter window sealant. Again, working with mechanicals so I've not played with the bodywork much yet.