The four Southend PD3/6's converted to open top were normally employed on the 68 (Thames Drive to Shoebury East Beach), with co-ordination area partner Eastern National working the 67 (Kent Elms Corner to Temple Sutton). However, in a number of years EN were not able to staff the service and ST took over, hiring EN's KSW (and, latterly, FLF) open toppers.
Peter delighted in getting the "wrong" bus on the "wrong" route, ie a PD3 on 67 or a KSW on 68. Here he is about to reverse 312 into a cul-de-sac at the Kent Elms terminus of the 67.

Photo from the Peter Cackett collection, photographer Jack Wilkinson, taken Eastwood, Broomfield Avenue