The entry for 23rd January 1966 shows the 140 as operating via Porlock Avenue, so it appears to have been in force at least from that date, if not earlier.
I also looked up the 114 and 158. The 114 was rerouted via Porlock Avenue from 23rd January 1957 until 23rd January 1966, when the 114 and 158 swapped routeings south of Harrow - the 114 took over the 158's route to Ruislip Lido and reverted to Bessborough Road, whilst the 158 was diverted via Porlock Avenue to terminate at Rayners Lane. When the 158 was withdrawn from 30th October 1971 the 114 (which converted to driver-only SMS operation from the same date) returned to Porlock Avenue.
This left Bessborough Road unserved and there were calls for a service to be restored. London Transport's solution was to run a limited service of one bus per hour on route 114 via Bessborough Road instead of Porlock Avenue during Monday-Saturday daytimes, these journeys commencing from 18th November 1972. Buses operating these journeys carried distinctive black-on-yellow blinds. If I recall correctly it was only short journeys to and from Eastcote Lane that operated via Bessborough Road, I do not ever recall seeing a Ruislip Lido journey and I don't even know if black-on-yellow Ruislip Lido/Ruislip Station displays were ever produced. These journeys ended from 22nd April 1978 when route 258 was extended to South Harrow via Bessborough Road.