Got a round of 20mm Oerlikon the other day (Hstp (12,4,8)= 55,RG,OeZ) and posted the markings on the BOCN forum for ID. Here’s the BOCN thread
On the case wall just above the rebated portion there are the markings “SWN/262/56/HEI/TNT/N12Z” (and JohnMoss, those slash’s are part of the marking-I can see why it’s confusing)
Thanks to BOCN member ydnum303 for translating most of that for me:
55 = 1955 (year when case made)
RG = Radway Green (Ordnance Factory in Cheshire, where case made)
Oe = Oerlikon (the gun it was for)
SWN = Swynerton (filling factory)
262 = Lot number
56 = 1956 (Year filled)
HEI = High Explosive Incendiary
TNT = Explosive compound in shell
But as yet I have had no luck with the N12Z bit.
TonyE suggested N12Z could be Mk2Z but it is definitely N12Z (it is a definite 1 not l)
Could it be something to do with New Zealand e.g. When New Zealand purchased ships or equipment off the UK the ammo was part of the contract?