My search at Scotland's People showed only ONE death for a Robert Stevenson during that time frame. That Robert Stevenson was a mason (so that information fits) but the record stated that he was the widower of Janet Houston. Record says he died on Main St. in Beith, Ayrshire. I do not know if that is the same as Stewarton.
Anyway, it all seems pretty odd. I am just wondering if anyone has Robert Stevenson and Ann Jamieson as ancestors or knows anything more about them. Could he have had a double life or a double wife