Yep - It is indeed a "Mears Slide" . Used on the Vulcan (and I don't know what else) Invented, I think by Wally Mears - later OC 44 Sqn. During astro shots the the Captain flew, Co pilot did the timing, the Nav Radar took the shots, the AEO watched heading and speed changes and worked out corrections using said slide and the Plotter did the sums! the "TO" and "FROM" refer to corrections towards or away from the point on the earths surface below the body being shot.
Why can I remember that but not where the car keys are??