What does METAR show when actual RVR is below the reporting minimum? RVR is measured between 50m and 1500m (ICAO). When the actual RVR is below 50m, what does METAR show?
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)