Hello Tobias. No, it doesn't fit. Buying the FSN800 is one of my dullest buying experiences. Didn't study it enough in the shop. At home I unfortunately found out that the rail is not compatible with the PKS40. At least I didn't "get it together".

According to the Bosch website, the FSN 70 and FSN 140 are compatible with the PKS 40.

The FSN 800 is suitable for the Professional series (GKS, GKT, GST...), but not for the PKS40.