VIN starts with 5 , Torinos start with 1 99% sure 5***** is a Mercury VIN
According to all the sources I have seen, that is correct. First pic is from the 68 Cougar/Fairlane/Falcon/Montego/Mustang shop manual, second is from the 65/72 L-M MPC. In various other shop manuals, parts catalogs, and service specifications books, the example Mercury VINs always show 500001.
Mavericks were Ford, not Merc, and trucks used a letter + 5 digit number, so those possibilities can be eliminated.