Just did a quick search- seems nobody is quite sure of when 531c was introduced, or named as such, anyway..
I remember the '70s as being 531db, with 753 & 531sl from about 1975/6?
My memory tells me that sometime- I reckon no way was it before Thatcher got into Downing Street- 531 got divided up into 'Professional', 'C', and 'ST', each tubeset getting a different decal. Before that, 531db had always been available in different gauges, I think, so really it was just a bit of marketing- typical of the '80s... I could be wrong!
Keithglos- was that Whiskers in Kilburn or in Goff's Oak? May have been somewhere else altogether in '71... Before my time. Whiskers got the lion's share of my paper-round money in the second half of the '70s. I've still got a lot of the stuff I traded it for on my bike right now..