What exactly does the headstone say?
Noting that William and Janet's younger children were baptised in Airth, could she have been the Janet Smart baptised in Airth in 1750?
There is a baptism of William Jaffery on 29 August 1730 in St Ninians. If you search Scotland's People for Jaffray you wouldn't find that.
But that William J*ff*r*y would have been 60 when Janet Smart's youngest child was born, which also looks unusual.
There's a baptism of another William Jaffery in St Ninians in 1746, who looks like a possible candidate.