I'd hazard a guess that the blind was being wound to (rather than from) 649, which at the time was the route to Rough Common, in peaks only, and included a regular single-deck working. This came in as a 652 from Chartham (St Augustine's Hospital) and went out at 1610 on a 649, which departed from that stop as it was in the same general direction as a 604 via Whitstable Road. 1610 would have been about the right time of day for the picture.
I attach a diagram of the bus station layout at Canterbury, as it appeared on a map in one of the last timetable booklets issued in February 1980 shortly before everything went over to leaflets.
Derek J