Yes, probably as sub-bus.
Service 15 was introduced in 1997 as a Eunos - Pasir Ris bus, which
had an arrival interval of approximately 17 minutes.
It was amended to Marine Parade Road sometime in 1998, I believe.
This resulted in an increased frequency (it merged with M4 and thus
had to have M4's frequency) and Volvo buses were used in addition
to the occasional Nissans, which had almost disappeared by
then.