Hello Stan… Thanks for your further advice. If indeed the front gardens are pebble coated for cars, clearly the garden and the ‘rock’ in question have been cleared away. The only hope might be that the rock was kept or referenced to some authority because it was unusual, and so there may be a trail or someone might remember. (Not very likely now after 59 years!)
On the subject of staying in the one property, one of the reasons we searched for our present property is that I could build an observatory in the backyard with clear east and west horizons for my lunar/planetary observations – and still be close to the city for my work. Quite a ridgetop site. See photo. The house is behind trees to the right from this angle.