RVR is given only when horizontal visibility (or RVR itself) is less then 1500m. If RVR is more than this 1500m. a P is added in front. Here R24/P1500.
When letters are shown after this it indicates a trend over a period of 10 mins, as correctly said by Red Four.
"There is also a code for staying the same (which I cannot recall at present)."
It is then P1500N