just some thoughts; the Navy liked electric primers and were in some ways more advanced technically than the Army maybe because they needed to be more engineering orientated due to engines etc and on large guns electric gave central co-ordinated firing which helped in using the fall of shot to adjust aim. The 12 pr gun was mainly on destroyers in 12cwt and 18cwt form (cwt is the gun weight and from that the strength of deck required for mounting and taking recoil) the 8cwt I presume was for smaller ships armed trawlers and yatchs. The 4cwt did not have a protruding primer and was for a landing gun like that used in the army/navy gun race. As to a 6cwt don't know. There was also a 12cwt fixed round used in anti aircraft guns which is 15mm longer than the SL case, has no cutouts and has crimping to hold the shell.