I don't know whether you have had many responses yet to your appeal about the mystery items. Here are my suggestions.
The black box marked Truro. Many of the smaller stations had a square box to send under lock and key to Truro to pay in their weekly takings. The boxes were signed for by the guard and conveyed to Truro each week for banking. However, the banking boxes that I have seen had a big padlock on them - this box pictured seems not to be secured so effectively. Perhaps it was for returning non-valuable items instead - routine paperwork, perhaps ?
The oval plate is certainly an LMS bridge plate. I used to own a near identical plate, which was numbered 125. That came off a bridge near Penmaenmawr on the north Wales coast. I would assume that bridges along the coast to Holyhead probably started with no. 1 at Chester (or maybe Crewe). I think that we can safely say that there wouldn't have been sufficient bridges west of Penmaenmawr to reach no. 321, so it would probably have come from a bridge on the West Coast main line.
I hope that this is of some help. Best wishes, Peter
Many thanks, Peter
A short report.
Many thanks to those who assisted in this venture - namely Tony Wright, our liaison with the South Devon Railway, John Ball who organised the coach, Roger Winnen who collected the names and planned the coach route and collection places - Keith Jenkin provided a special headboard and notes on our much respected former chairman.