Amazing piece mate!
Yep, this old girl could tell some tales alright, been around the block more than a few times - literally!
Well mate I thought you'd be hard pressed to find any shells - apparently not!
https://www.rebelgunworks.com.au/collections/shotshell/caliber_28-gauge

You weren't wrong about the matching rib repair - that is brilliant work indeed!
As to converting back to a rook cartridge, that would be a tough one, depends if you already have something similar, these do turn up from time to time though!