Jean Marie, thank you for those files. It is very helpful to see the description of the shoulder belt plate dating from 1815-20.
At the risk of seeming ungrateful and, rashly, I didn't anticipate any possible confusion, but it turns out the article I am after is a separate piece by Dixon Pickup relating to the 58th entitled:
A RARE BELT PLATE OF THE 58th (RUTLANDSHIRE) REGIMENT
Fragments ('snippets') accessible online read :
"The most important feature of the plate is the display of the Castle & Key from the Arms of Gibraltar; it is not too sweeping a statement to say that this is most probably the earliest appearance, albeit unofficial, of these emblems on a British military badge."
https://books.google.co.uk/books?id=...me&q=Gibraltar
Would I be pushing my luck to ask if you have access to this article- ? pp.x-xiv as well?
With thanks,
JF