I bought an old Rossin road bike thatīs been converted to a fixed gear bike. I was wondering when the bike was made. The quite exotic paint job seems to be original, and might be the best clue. There is an "R" engraved on the fork, but the only engraving on the frame is "52" in the bottom bracket. There are no markings on the seat stays.

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I know nothing about Rossin but my guess based on the paint style is '85-'90. I'm sure you'll get some other choices, too.
That might be an SR Laprade seatpost and they are dated below the insertion line. It's possible that's it's original to the frame and may be of help.