Hi Rob,

I, too, was wondering why no "S" as part of the serial number.

Doing a bit of WAG'ing...maybe it was a retro-fit kit that was available for Royal-authorizied shops to offer a customer used to having it on the Royal HH's...and the modification was done in the local shop...???
 

I'd consider it an example of a custom ordered model rather than one that was 'modified' later on. We can't lose sight of the fact that many factories offered special or custom options, and these machines clearly don't turn up as often as the off-the-shelf models.

According to David Ford, who was a service manager for Royal, there was an FP variant produced in 1961 called the Britannia. He noted that this model had a bluish colour and had hand tabs, hand margins and the same ribbon spools s the HH. He believed that it was only sold in UK. 

David also recalled that FP models varied depending on where they were made. For example, FP models made in Canada were built from different materials, they had black key levers instead of chrome plated ones, and its touch was lighter. It was considered to be a better machine than those made in the Netherlands.

Regarding the serial number prefixes, you’ll find either a FPP, FPE or FPS before the serial number.

FPP models are 10-pitch with pica type. These machines could also be fitted with an elite type face at customer's request.

FPE models are 12-pitch with elite type. 

FPS represented special type, and could have a standard a type face, split typeface or hand engraved typeface. FPS models could have been set up with 5, 10, 12, 16 or 20 pitch, and almost any type face.
 
FP typewriters were available with 11”, 13”, 16”, 21”, 27” and 33” carriages.