Looks to be an interesting link Jas .
I just scanned through it real quick . Will read more tonight
Im not sure the left mark would be the BPC mark . Those that I have seen were very clear in their lettering also the crown is normally very much more defined and detailed. In doing so they are often not as deep as what we are looking at here .
also the BPC at least on those that i have held , was very much sperated from the septers . so even when they were corroded or in one case double struck , they still did not bleed into the septers .
Ill look , at one time I had a set of barrels out in my shop with the BPC marking on it .
But I do believe that what we are seeing on this barrel set is 4 marks , not 3
As to that left hand mark . The question at hand , at least for me is if that mark carries the V or not .
Frankly to me , it looks like it does . But then we are looking at a photo so
But if it does then it would IMO be either the proof or the definitive proof . This making the reversed L shaped Box on the far right that’s just visible in the photos , the viewers mark
PS: we also cannot pass over the detail of the crown , which IMO takes it out of the earlier marks or other national marks