What does METAR show when actual RVR is below the reporting minimum?
If the RVR is less than the minimum value that can be measured , M will precede this value e.g. R24L/M0050.
  


If measured in ft, then FT will follow. For example: R24L/M0200FT

For greater than can be measured and trends:


  
  If the RVR is greater than the maximum value that can be measured, P will precede this value e.g. R24L/P1500.
  If RVR trends can be measured then U, D, or N will follow the RVR value to indicate increasing, decreasing or no change respectively.
  




Source: https://www.skybrary.aero/index.php/Meteorological_Terminal_Air_Report_(METAR)