The tag said No.4 Mk.1......is it?

[Peter Laidler]

That's why, when we fit a new bolt, after blueing and checking the locking faces of the bolt and body or bolt head, we smear a light thin coat of engineers blue on the rear face of the NO-GO CHS gauge and LIGHTLY close the bolt/bolthead down onto it. There MUST be a full impression of the rear of the gauge on the face of the bolt