Peter Kerslake says "It is worth noting that the smoke deflectors fitted to 70020 Mercury have no backing cup to the four hand-holds. This type of handhold was substituted for the earlier handrails when the engine was going through Crewe Works, and followed the derailment of Polar Star at Milton in 1955, the accident being in part attributed to poor forward vision for the footplate men when the original handrails were in place. Five engines were so treated.
I think that this is the twelve noon Penzance to Crewe with Old Oak Common's 70020 Mercury ready to leave, almost certainly having replaced the engine which has worked up from Plymouth. The leading Royal Mail Travelling Post Office coach from Plymouth has also been removed".