Is this a Mk5 bolt

[Surpmil]

The long slot in the long recoil lug is standard to all No4 and No5 rifles and originates in the No1 MkVI if I remember correctly.

As for the bolt knob hole, some No5s had a considerably larger hole than the No4s, and the knob was also shortened somewhat. The hole sizes were about 3/8" versus 1/2" as I recall.

I doubt the hole was ever used for work-holding during manufacture, if only because plenty of other bolts (No1 No4) were made with no hole at all)