The No 4's had numbered bolt heads. The No 1's were adjusted the same way, but the bolt heads were fitted to the gun and not numbered. You can still find used bolt heads, but you need to have a measurement of yours, and know what you want to do with it size wise to take up headspace. Even with headspace in spec, the chambers are loose, and cases bulge and stretch. Neck sizing only makes a big diference in case life. There really isnt a good reason to full length size the cases if you shoot them in the same gun, so long as they will go back in the chamber. Still, just sizing enough to get them to chamber will make them last longer than full sizing.
Any decent gunsmith can set the barrel back a turn and rechamber to tighten up the headspace if it truly needs it, but most of the Enfields still look like they have headspace problems when they don't.
If your receiver is peened where it locks, it may in fact be worn out. Get a good gunsmith to look at it before making any decisions, or get good clear pictures of what you're concerned about and post them.
Take a good picture of the case also, what you're calling immenent case separation may in fact be they way most Enfield brass looks, a bulge ahead of the rim, and somewhat egg shaped. The British military didn't care what the brass looked like after they were fired or if they could be reloaded, they cared about reliabilty in the field. Loose chambers were part of the plan, even if headspace was in spec.
] out of the rifle).
