Dave, I don't have my filter cap handy to check, but I want to think it may have a slight taper to it so that either size will work. My 76mm seems to be a bit deeper than the average filter socket. I can say mine is a very tight fit as it should be. I like to use it so it can be torqued to proper tightness. The torque value is molded in the cap so it must be important since it is of plastic material.

James