The seat post diameter could be a clue.

I had a recent exchange with Mercian about my '83 King of Mercia Touring because I was having a hard time with the 27.2 seat post. They confirmed that in the early 80's, the KOM Touring model would have been reamed out to 27.0, not 27.2, and sure enough, a 27.0 post fit like a glove.