Birmingham guns to be retailed by a London firm were often proved in London as 470evans demonstrates.

Even the opposite is true I believe. The details are not to hand but I have a recollection of reading about a London maker/retailer having a disagreement with the London proof-house director or staff, and subsequently sending his guns north for proof! Anyone shed further light on this?

There is only one hard-and-fast rule when discussing the British gun trade:
Never say "never"!

BTW, my memory is not too bad but I and others might benefit from your re-posting the photo of that 16-bore, save us from going looking for it. This might draw something from the newer members as well.
Thanks in advance.