The letters on Remington barrels are a date code.
First letter is month of manufacture, on the cipher BLACKPOWDERX. Yours is month of April (C).
Second is a sequence for the year. Letter P could be for 1923 (not possible considering the 552 wasn't made until '57), 1967, 1995, or 2018. Given that you seem to have decided due to other features that it's at least 40 years old, that only leaves 1967.