I've renamed my electronic copies of the various surveys! :lol:
The monotone 'Cwmorthin Slate Quarry Underground' dated 16/08/92 is what I consider to be the best version, although the three-colour version of the same map is probably better from some points of view.
I'd agree with you that the Second Edition maps are of good quality and I've always considered them to be 'fit for purpose' as far as navigation and exploration is concerned.
The real gems are the 1884 and 1899 surveys, although they require a bit more work to understand them properly - not quite wet towel round head territory, but close ;)
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Becks - Speedycaver's surveys are worth a look too: they offer a different way of looking at things which can be quite useful for looking at some the detail.
http://www.aditnow.co.uk/mines/Cwmorthin-Slate-Mine/
Hello again darkness, my old friend...