Wait.... the "1976" bicentennial PROOF coins is listed as a 1975, NO S mint mark. if I recall correctly. Its called a "1975" because the penny and dime have the year 1975 on them.
That was a year for the error on the 10 cent with NO "S". That one is pricey.
If it is extremely rare it should be listed in the
Cherry Pickers Guide, but I dont have access to my copy at the moment. If some one does, it may be listed under 1975 or 1976.
I know that some of the "varieties" relate to the type of mintmarks used....In the CP guide, check the back of the book and there is a few pages that show each type of "mint mark variety".
Typically no mintage numbers mean that they don't know the total number, and the numbers are simply part of the normal coins that were minted for that year.