This is a London hallmark with the date letter P for 1890. The maker's mark is CS FS in separate boxes for Saunders & Shepherd (Cornelius Saunders & Frank Shepherd) although only the FS survives in recognisable form:
The handle of the spoon seems to have been re-formed at some point after the hallmarking, possibly converted from a rectangular to a cylindrical cross-section.
p.s. I note that your thoughts have beaten me to the solution, but I'll still post what I have composed.