Hi Andrew, I have 1318FS and have also seen 1336FS. I don't think there's anything special about the FS series of numbers (except for them all being RRAs of course), I think it was just an additional block of numbers extra to a certain years production after all that years numbers had already been allocated. Raleigh's serial numbers are quite messy, a lot of their records were destroyed in a fire so their archives are very incomplete. I've been through the index of archived documents now held at the Nottingham City Archives and there's no list of frame numbers available covering the RRA years.