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Rommel you wrote:

CF and DF where the  F signifies the line is folded over.  That is to say C and D both begin and end with Pi.  Of course! The penny drops.  F meaning Folded means moved along a by 3.142 or effectively multiplying by pi.

 CI and DI (not 1) Are inverted, which is to say reflection of the C&D scales.  Inverted, yes!  How blind of me.  I've taken another look the 1s and the Is are completely different.

CIF is a funny scale in that it is inverted and folded and the left index is 3.5 and the right index is 3.1  on the K+E Log Log Duplex Decitrig 3.1 is thus the right index in that it extends past what would be the traditional place of terminus for the C or D scales.  If you have a slide rule that has an inverted and folded C scale (CIF) look at it.  Interestingly, looking at Bob Goethe's photos, you can achieve this scale in two ways.  Upwards, by taking CI and F ing it, or downwards by taking CF and I ing it.  The laws of algebra I suppose. You need to keep a careful track of the decimal point though.

This site http://www.sliderules.info/a-to-z/scales.htm  Does a pretty good job of describing the scales on a just about all scale rules out there.  You'd think I'd have learned by now.  If you have a question, Google it first. 

 That's the great thing about this site; you learn something new everyday.  Conversly, you realise you don't know anything like as much as the tiny amount you thought you did know.  DaveP