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Crows-
The 27 follows that the 531st frame tubing was special tourist and a little thicker. Looking at an 80s Mercian King of Mercia and it isn't to different (particularly the top routed brake cable) apart from your lack of rear derailleur cable stop and BB routing. Maybe it was removed? Mercian (or likewise) do custom stuff on frames so that could explain the choice of dropout too? Just guesses...
Squaredisk
Also seatpost size might offer clue to age?