Question About 1968-S JFK Half & PCGS Ad In Numismatist

On p27 of the March, 2008 issue of Numismatist there is a full-page ad taken out by PCGS. It shows stacks and stacks of faux-PCGS slabs, many showing impressive coins inside of them. The images of the coins are likely from the in-house TruView service and they are photohoped onto the holders. However, one holder shows a 1968-S JFK half that is graded "MPR70DC" and with the certification number 7733372. This certification number is a real number that corresponds to a PR70DC 1968-S JFK that has its own TruView image online. My question is; what the heck is the "M" for in "MPR70DC"? Is it simply a typo in the ad?
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Russ, NCNE