[CR]Reynolds 531c, what does the "c" mean?

I feel like this has been discussed recently, but I can't find anything in the archives.

Anyhow, yesterday I collected a couple of bikes from a kerbside rubbish collection that I cycled past on my way to work. One of them is an English made Raleigh, 56cm c-c, white and blue, with a sticker at the base of the seat tube that reads: "Reynolds 531c fork blades, stays and butted frame tubes" and one at the top of the down tube reading "Raleigh lightweight frameset".

So what does the "c" mean in 531c? Presumably it is only single butted, otherwise it would explicitly state double butted?

From the components (not classic by any stretch of the imagination: Shimano Exage sport derailleurs, 7spd index DT shifters, 600 cranks with biopace chainrings, 105 dual-pivot brakes/levers) am I right in presuming that it would be from the late 1980s?

It's a bit rusty, needs a good cleanup, and only had a front wheel, but it seemed worth saving.

Thanks,

Mark.