Can somebody help me please with the identification?

[paul monfils]

I agree with Stramonita haemastoma (Thais haemastoma in some older books).  The problem here is not that the shells need cleaning, but that they are eroded as a result of washing up on the beach and being worn down by repeated rolling around in waves and sand.  Bleach won't have any effect on shells in this condition, and vinegar (or any acid) will just erode them further.