Vandervell did mark their No 5 bodies, unusual, as most didnt but they produced vast numbers of Mills grens.
The slab sided or channelled lever was very soon superseded by the flat and then ribbed lever and the 23 had the slotted striker[a few No 5s may have had this] but i cant see any 23s with the early lever. I suspect that the base plug has been changed on such grens. Such levers may fit 23 grens if enough are introduced to the body but i dont think you can tell from the body of the gren if its a 23/1 or 2 or a No 5.
I agree with you John, a base plug dated 7/15 would be worth a lot but a 5/15 or a 6/15 would be worth, well, so rare that i dont think a figure springs to mind, certainly a lot, However, there isnt a lot about, except for the fake Barbour plug on ebay right now. Bet there will be more of these, even 3/15 or earlier.
Andy