I think Erhard is on the right track. I have an identical style ring, also hallmarked for London, but date 1972. It has what I believe is a PY stone and this sits very neatly in the setting.
When I look closely at the setting of the stone in the questioned item, it does not sit as neatly as my example but seems to show a gap or ridge (?) between the edge of the stone and the metal. So yes, it may well be a replacement.
As for the question of whether a stone could have both PY and PH elements - in theory it could be possible ... but I think it would not have happened.