Wild Skies: I have gone back to your pictures and, for what it is worth, yes, I agree. There is no significant difference between the set up in skeettx's picture and yours. The rib is simply a 'swamped rib' but I would repeat that judging by the style of the shield, it is likely a re-barrelled or re-ribbed barrel set.
I have in stock a 16g H&H Royal with a swamped rib that was a retro-fit. On that gun the rib has been merged into the breech profile, only sinking away from line of sight after an inch or so.
I also have a 12g baraction Blanch hammergun which has the same shield as the Blanch you posted and this was obviously originally made as a swamped rib.
Bushveld: As to the style of the gun, I have restored several Purdeys from this era with standard backlocks, peninsular as you describe them. Certainly Purdey produced many island locks but it was not their only style. It is difficult to be definitive without handling the gun but the overall style and engraving looks like a Purdey. However the rib engraving doesn't look right: too bold and not laid out quite right. I would guess it was put on by an outworker who was not really familiar with the Purdey top rib engraving style or simply found the small lettering too difficult to reproduce.
