Well I didn’t check your measurements when I answered, and just assumed it was the 24 ga for the Snider. All of mine (so-called Snider) are headstamped 24 ga for what that is worth.
So that said, sorry but I don’t have an answer for you. It does have a very unusual rim proifile & I apologize for not looking closer.
May I ask how you know Kynoch NEVER designed a 28 Thick Rim?
As to the Snider conversion: I don’t know the time period, or who did (or how many different smiths performed) it. The Snider cartridge, having been made in all the different variations & types (Boxer & drawn) has variations in it’s rim (thickness and diameter), this forward tapered rim, on the so-called Snider shells should accommodate the various chambers with little gunsmithing. Re-doing various cambers would be much harder/more expensive than having a small order of shells made, OR perhaps Eley saw a market & offered / ran small lots?
I know this is not a satisfactory answer, and is only conjecture, explaining these tapered thick rim 24 ga shells, as we know Eley, Kynoch, Arsenals & Dominion offered drawn and Boxer shot shells for the various Snider chambers (Canadian & British).
Love to hear about these …
In re-reading your measurements you use “neck” as a point of reference, where exactly is the neck?