It most likely does have RIAA equalisation. ffss was Decca's series for Stereo records, as opposed to the ffrr Mono series. Deccas pressed before 1954 are more likely to have a proprietary equalisation, as RIAA became the standard around '54-55. The proprietary curves (Columbia, Decca, etc) were not hugely different from what RIAA ultimately turned out to be, so you're unlikely to hear any major differences. Then again, there are a few high-end phono stages with different EQ options for those who are a bit paranoid about these things.