How would SLP997 be calculated on a METAR?
Refer to FAA document, AC 00-45H Aviation Weather Services


  3.1.5.13.25 Sea-Level Pressure. 
  
  At designated stations, the sea-level pressure is coded in the
  following format: the identifier SLP,
  immediately followed by the sea-level pressure in 1 millibars (mb) .
  The hundreds and thousands units are not coded and must be inferred.
  For example, a sea-level pressure of 998.2 mb is coded SLP982. A
  sea-level pressure of 1,013.2 mb would be coded SLP132. For a
  METAR, if sea-level pressure is not available, it is coded SLPNO.