H&K MP5: What Does S/E/F Mean? Posted February 8, 2017 in NFA / Suppressors / Class III by Nicholas C with 21 CommentsTags: H&K, Lower Receiver, mp5, selector lever, selector markings Have you ever looked at the older MP5 lowers and look at the selector markings? It has the letters S/E/F. Here is an explanation from H&K. Sicher – SAFE Einzelfeuer – Single Fire Feuerstoss – Fire Although this photo explains it rather well.