Originally Posted by
Bruce_in_Oz
Engaged the old brain and dived into the "digital crypt".
It appears that this "dodgy" safety is the "Safety, Expedient", a notorious "lash-up" to get rifles in the hands of troops. Allegedly supposed to be withdrawn and destroyed as "proper" ones became available. (See my previous comment about the "engineering" involved in cutting the 8-start thread on a "proper Bolt, Locking).
I was under the impression that these were produced for the No.4, hence the grooves for the No.4 style spring on the exterior, as opposed to the little holes on the reverse side that engage the hard steel pins fitted to a SMLE receiver.
Any other suggestions?