In Tom Noble's excellent 'Diesels On The Regions - Scottish Region' there's a photo of a 37 with a single VAA or VBA captioned thus -
"37081 arrives at Crianlarich on 23rd August 1982 with mail from Fort William, for transfer on to the 17.58 Oban to Glasgow train. The locomotive and van will return to Fort William, uplifting any traffic that may be lying at Crianlarich. This arrangement followed the suspension of the 15.20 Corpach to Mossend and the corresponding 13.38 Mossend to Corpach trains."