Top shows a mark 8 lens, blue/grey coloured frame with small "tucks" in the frame at the bend
bottom shows a proper mark 7 lens, more of a royal blue frame with no "tuck" on the bend
top shows a mark 4 lens in tow pieces no frame
Bottom shows a lens from a mark 4b lightwieght originally black frame with location lug
this shows the back of the mark 4b lens showing it to be compleatly different to the later type lenses.