And in related matters:

NEVER remove material from the front face or the rear of the threaded tenon of a bolt head.

Not only does that overall dimension "set" striker protrusion, but if you remove the VERY thin hardened surface layer, you may get other problems.

There are two types of material used in SMLE bolt heads: malleable cast iron and, "mild steel". If your bolthead bears a little "M", it is made from "special" mild steel and has been hardened by carburizing the surface skin. Thus, this important pressure bearing component has a "hard" skin and a relatively soft core that SHOULD never shatter in the event of a catastrophic "Ka-BOOM". The right side rail of the receiver (body) may stretch a little and distort, the bolt will probably skew sideways towards this distortion, but the firer will be "shaken" but not requiring urgent medical assistance.

Could it be that the "dust-cover" on early Lee Enfields was also there to deflect errant gases and particles in the event of a failure of the early brass cases?

"Plain" bolt heads are made from malleable cast iron, (which is nothing like the "cast iron" used for the iron lace-work on old houses), a "low-ish" carbon, iron alloy (NO fancy stuff like chromium, manganese, vanadium, etc allowed), that can withstand the sudden shock of cartridge ignition and wear from all that "action".