ScottS - the Jones underlever - all other things being equal is the the stronger. Firstly - the action bar is is nearly completely solid and hasn't been "milled" out for the sliding bolt of the Purdey bolting system. The clamping force generated by the Jones system basically locks the barrel flats to the action bar - guaranteeing virtually zero movement during recoil. You may deduce what the inventors of the Purdey system thought of underbolts by the fact that they used a modified semi-snap action rotary system on their early nitro doubles. Also, the English used the Jones system for most of the heavy nitro doubles (577/600) that they made. That underbolted hammerless designs were later adopted for those heavy rifles is probably more a reflection of the better steels available just before, and after WW1.