Has there ever been a US airport with identifier PM2?
No. PM2 would not be a valid ID in the FAA system. According to the FAA order defining the system:


  1−2−7. ASSIGNMENT SYSTEM §c. Most one−number, two−letter identifiers have been assigned to aviation weather reporting and observation stations and special−use locations. Some of these identifiers may be assigned to public−use landing facilities within the United States and its jurisdictions, which do not meet the requirements for identifiers in the three−letter series. In this identifier series, the number is always in the first position of the three−character combination.
  (emphasis mine)


So two letters followed by a number is not a valid ID.