Comment by albert:
I finally had the opportunity to do a side by side comparison of the original turquoise/cyan cover with a later 1970s pressing, see the new pic. Thank you to Ben at The Little Record Shop for allowing me to take pics.
My idea was to use photoshop to analyse colouring on the back cover sepia style photo, to see if cyan is in the mix. Unfortunately results were inconclusive, cyan registers very low on selected areas of both covers and may only be there due to the lighting when the pics were taken.
However, you may be interested to see how far the image degraded after only a few years. On the original cyan sleeve the back cover (right) has a nicely clear and detailed image, whereas on the later pressing (mid 1970s) the image is faded and lacking in detail.
On my own early pressing the back photo image looks identical to that on the cyan sleeve, it is clear and detailed, ...which is a disappointment as I was hoping to see at least some degradation in the quality.