Hi KeMac,
Afraid I can't answer your question, but you may remember that 1968 was a turbulent year for BUA that led, among other things, to a pilots' strike in Jersey, followed by the demise of their BUA(CI) (formerly Jersey Airlines). The flights you're interested in could have been affected, as BUA itself also suffered some industrial disruption.
Just to clarify your question:
(1) Was the flight into SOU operated by the same a/c as the SOU>GLA>EDI service you used, and was the flight number the same? Or did it merely enable transiting pax to connect with the departing service?
(2) Was the flight into SOU from JER via GCI and EXT, or from just one of them?