I've got an even BETTER idea................. If someone was so stupidly unwise as to tell you that he considered that it was indeed a rifle bolt, you could just say that he was right........ It was a bolt for a .410 shotgun. Or even beter still, one of those No4 converted AIR rifle thinggies........... Anyone remember what they were called now? I'll give you a clue........ The initials were S&P........
Back to the gauge bolt and reality. I have a feeling that those gauge parts were adapted/calibrated initially by (was it ?) Coventry Tool and Gauge and the bolt was very probably hardened through to give it a better chance of surviving before being accepted by the Ch Insp Armts. I am sure that this'd be the case because you couldn't have a worn out bolt being used as a gauge for er......., another worn out bolt! We had these part gauges for the Vickers and a gauge, Inspectors Piston Post and a Gauge Inspectors gas plug for the Bren too.
NOW.......... anyone who has one of those (the Gauge, Inspectors, Piston Post will come up against the same problems.............