N67/051.
I suggested following explanation:
When I was driving and unable to match cloud continuously. At one point when the cloud broke up at the sky, the bird left sight of the original small white cloud and the eye was caught by the forming vapour trail of a high flying jet. That is, the vapour trail was just beginning and was of a similar size and colour to the original little cloud. The elongation and motion was simply the configuration of the aircraft's motion at the end of the vapour trail and of less rapid diffusion and disappearance at the rear end of the vapour trail.
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OVERHEAD
GAL. 104 METEOR CENTRE NATIONAL RESEARCH COUNCIL OTTAWA 8, ONTARIO
National Research Council of Canada/ RG 77, Vol. 306
Conseil national de recherches du Canada