The 1936 Proof Mercury Dimes were only produced in a Brilliant finish unlike the cent and Nickel Proofs from 1936 which came in two different finishes.
PR60-63: Several hundred examples may exist in this condition and these are coins are the ones that were probably cleaned or stored incorrectly.
PR64-65: Most 1936 Proof Mercury Dimes are in PR64–65 condition. Possibly about half of the mintage exists in these two grades combined.
PR66: In PR66 condition it becomes somewhat challenging to find available in this grade. Possibly no more than 400 coins survive in this grade.
PR67: In PR67 it becomes very difficult to find and probably no more than 100 examples exist in this grade. In PR67 condition very few examples surface at auction every year (no more than 3 to 5 coins a year). So, patience is really required when trying to find one in this grade.
PR68: This is the highest grade available and PCGS has only graded one example in this grade with none being finer.