the transavia 737 was an ADV model so maybe BY used it subbing it for LBA flights due to the short runway if it had -15 or -17 engines
BY did have their own ADV models by the early 80's
afaik but they may have all been being used elsewhere - JMK JTR and JSI spring to mind really needing ADV models at the time
I was with MON at the time (1986) and our 4 737's then were all ADV