For my reference, does a world renowned mint strike MS-70 coins off the press? Or is it a mixed bag or MS-68's, 9's and potentially no 70's? Also, what is the check and balances for attributing an MS-70 rating? It seems as though there's such a fine line between 69 and 70, no? What's to stop a collector knowing he's got a coin of superior quality and buy an MS-70 rating over a 69?
No mint grades their coins, they all come off the assembly line as-struck and usually put into either a "mint tube" roll of 10/20/40 (depending on coin size), or put into capsules (they will charge extra). There is no way to know how any coin, round, or bar will grade from the mint, they are all put through the exact same minting process. Usually you will have a variation of anywhere from MS-67 through MS-69 in most tubes of modern pressed coins (government or private mint), with a very few getting the 70.
All grading is done by a 3rd party Grading company (TPG). The "Big 3" are PCGS, NGC and ANACS. ICG, SEGS, and some others round out the lower tier.
Each grader is human, and while there are written and pictoral guidelines, any grader can slightly differ from the other. Most of the top TPG's use a trio of graders per coin, with none of the others knowing who graded before them or after, each coin raw, and no notes shared. Each coin goes under a loupe/magnifier at a certain X number (I think it's 30, can't remember) and any imperfections are noted and count towards the grade.
Each coin usually spends less than 10 seconds with each grader, where they have to find imperfections and notate everything. Think about that. 10 seconds or less.
There is such a fine line between a 69 and 70 - some 70's could be 69's and vice versa. There are many stories where people will re-submit a 70 and get a 69, or a 69 and it comes back a 70. Most non-professional graders can't tell the difference between them, and with pros, you will also vary widely.