This is probably a stupid question but I have an opaque FM67 filter for my M2 and a red-filter FM15 for my M3. Now the rubber on the FM67 seems stronger and more secure than the FM15. The rubber on the FM15 is more flexible and is not nearly as securely glued to the plastic of the filter. Is this normal? It is just that the FM15 doesn't feel quite so solid as the FM67!