FP serial numbers used one of three prefixes:
FPE for models fitted with an elite typeface
FPP for models fitted with a pica typefae
FPS for models that had a special order typeface

Some models' serial numbers included the carriage size too, which the machine in question doesn't have.

Given that the typeface fitted to a FPS model was specially ordered, there could be any one of a large selection of typefaces options in that machine. I didn't read the thread, but it would be interesting to know the pitch of the typeface as it might help to narrow down what the actual typeface is. For example, if it's a very large pitch it's probably the Bulletin typeface.

It certainly isn't strange or that uncommon to find a Royal standard with just an uppercase typeface. Such machines were used for any number of specialty tasks, very often for billing and labeling.  

I didn't realize there were pictures, which of course provide more information. Grinding down the slug face is a bizarre thing to do but I have to imagine that there was a good reason for it. It looks like it was originally fitted with the Double Gothic typeface, which provides uppercase type in two sizes, and perhaps its owner wanted to make sure that the larger size was never used. This wouldn't be out of line if the typewriter was dedicated to filling out a specific form or cards and you didn't want a typist to mix type sizes on them.

Of note, it looks like the wrong ribbon spool has been installed in the typewriter. It should have the Royal spools that actuate the auto-reverse mechanism, but the spool that's in there is so small it looks like it might have been meant for a calculator or adding machine.