Hey!
I have seen and have some old classical Decca LPs with that "ffrr"

If I remembered correct it stand for "Full Frequency Range Recording" But even that I doubt that it is a true full 20hz-20khz recording.
Yes, likely from 50's. The LP will play fine on your modern turntable
Anyway, different record companies use different name for their recoding. Like RCA Victor use "Orthophonic" and later "Living Stereo" for their early stereo recording.

These old records are old but that doesn't mean that they won't sound good. Actually many of those old recordings would put most today's modern albums both on CDs and LPs to shame!

If you like classical music look for early stereo recording LPs... clean one from Decca, RCA, and Columbia and you will be amazed of how these old stereo recording sound. This will make you scratch your head and wonder whatever happened to the "recording art"

Take care,
Buddy