At Callicles request, here is my analysis.
More than 30 years ago, when I was doing my studies about distinguishing CS-made Bormann fuzes from US-made ones, I noticed the big "gasket rebate" (groove) at the bottom of CS-made Bormann shells, and began to distribute my theory that its presence was a way to tell CS-made Bormann shells from US-made ones. Full confirmation came via the flood of cut-in-half Augusta GA cache CS Bormanns. Every one of those that I've ever seen has the gasket-rebate.
So, the "guideline" I formulated is, the gasket rebate means CS-made, and no gasket rebate means it's either yankee-made or 1861 CS-made (before the gasket-as-premature-detonation-remedy was adopted).
That "guideline" has held true through all the following years, for every sawed-in-half Bormann shell with an identifiably CS-made or US-made fuze in it. Callicles' sawed Bormann is the first I've seen which doesn't match up with the guideline. So, I have to think the simplest answer is that his shell is a CS-made Bormann with a captured US-made Bormann fuze in it. EDIT: Or, a captured CS-made shell fired by the yankees with a US-made fuze in it. (There is documented proof that the yanks used captured CS ordnance during the Vicksburg Campaign.)
Background info:
As is already known, the Confederates had bad troubles with their Bormann fuzes detonating the shell prematurely ...often in the barrel or muzzle of the cannon upon firing. (After the battle of Fredericksburh, that problem caused Col. E.P. Alexander, Lee's Chief of Reserve Artillery, to ban their use by the Army of Northern Virginia. (Nonetheless, some still got used at Gettysburg ...and much later in the Western Theater, due to desperation.)
One remedy the Confederates tried for overcoming the premature-detonation problem was to manufacture their Bormann-fuzed shells with a deep groove (which Machinists and Engineers call a rebate) encircling the bottom of the shell's "main" fuzehole. The rebate's purpose was to accept a wider-than-normal leather gasket underneath the Bormann fuze. The theory was that the prematures were caused by firing-blast flame geting past the fuze's short threaded section and going underneath the fuze, thereby getting access to the shell's bursting-charge. You could say that the oversize gasket is a "headgasket," to seal the bottom of the main fuzehole from intrusion by firing-blast flame.
Important note:
US-made Bormann fuzes also had a leather or rubber gasket underneath the fuze. But the US gasket was no wider than the fuze's body. Thus there is no gasket rebaye in US Bormann fuzeholes.
Here are some close-up photos which show the difference I noticed long ago between a CS-made Bormann fuzehole and a US-made one.
Regards,
Pete