Well there are 12-bores and there are 12-bores! My .577 x 2 3/4 Light Nitro is built on a sleeved 12-bore shotgun too. No problems whatsoever with chamber-wall thickness.

I should add that the Greener 'Empire' is a massive shotgun with a long action-bar. The Brits distinguished fairly well between 'game-guns' and their 'pigeon-guns' or 'fowlers'. The latter two are usually heavily built, long-chambered, and proved for heavier loads. Many early pigeon-guns even had 3-inch chambers. This distinction is not always clear-cut in Continental or American guns.

Furthermore, a 12-bore Greener 'Empire' is about 3mm wider across the breech-face than the 10-bore Zabala! True! Might pay to put the calipers across that Centaure before making a decision, though the one on GB sure looks wider! I'll measure my Greener's chamber-wall thickness on the weekend and report back. I have three Empires BTW, two of which are still 12-bore shotguns!