Well,

The rifle is in hand. Its a James Macnaughton and it turns out to be a variation of a 577-500 No.2. Not sure if the "40" stamp makes any more sense now. Its got a 7-groove bore and near as I can measure its .506 - .507dia. That seems undesize. ? ? There is a strange "star" stamp on each barrel above the chamber. Anyone know what those means? (see pics)

Anyone have any suggestions or comments?