The GCD actually has these listed as all grouped together when they should likely be separate series, because the indicia for them doesn't have "Movie Classics". We just followed what Overstreet had done. There's actually a current discussion going on on the gcd-main list (which is a google group that you can join to see the archives... the discussion is called "Dell Frankenstein question".).
I do know (or at least I think I remember) that the last three digits of that number are the year and the month. So 312 would be 1963, December. and 501 would be 1965, January.
Also, glancing through a few, it looks like the first two digits are 12 when it's a 12 cent comic and 01 when it's a 15 cent comic.
And for some reason, organizing them by the middle three digits puts them in alphabetical order (rather than chronological order)...
Peter