I am also in the line of Alexander and Lydia. Robert's middle initial is indeed P as noted in the deeds recorded between him and his father in the Washington County records.
I believe that He is buried in the Court St cemetary in Machias. I was up there this summer. I thought i got a picure, but i cannot find it. His father IS buried in that Cemetary and i did get a picture of that headstone.
Julie Proctor Gibbs