Hi MG Mike,
In our terminology, as used in the Armourers workshops, the breech block stope were the beating surfaces on the left and right hand side of the bolt way that stopped the forward movement of the breech block on a what we call a 'dry gun' That is with no ammo. The breech block hits these stops and gradually wears/beats them down. On a live firing gun, the cartridge in the chamber indicates that that's as far as the breech block will go.
The barrel bearing is the ring in the body that the barrel seats firmly against when the barrel nut is tightened down. There was a repair at Base Workshops to bore this area out and fit a new barrel bearing. Worn barrel bearing was one that wouldn't allow a barrel to be fitted tightly even with the No6 (?) barrel nut. The .303 BRen had barrel nut spacers to correct it but, alas, not the L4 guns.
Quite why, nobody really knew!!!!!!! The barrel nut spacers certainly would fix the problem.
As a general rule, taught to us, the spacers could only be used on one of the barrels of the pair. If both barrels needed spacers when the No6 barrel nut was fitted, the gun was ZF. Fitting barrels and breech blocks and CHS was a bit of a nightmare to be honest, especially for those Armourers brought up on the Bren that suddenly found that the same criteria couldn't be used on the newer L4 guns......... that were really just old re-worked Brens!
The same sort of criteria applied to our later L3 Brownings........ which were really our old M1919's internally re-designed by Biffo-the-bear one evening after a drunken party in the woods.................