It certainly looks like Peter, and the thread you've found is intriguing, but the first part is still defeating me. As well as your suggestion, I wondered if it might mean something like 'Mrs', with this being the mother of the person being addressed below, but I'm not confident enough in putting an age to the sitter.
If no-one else comes in on this to offer an opinion, I'd be tempted to start another thread titled something like 'Date of portrait & age of sitter', give a link to this one, and invite people to comment here (not in the new thread).