If you really care, milsurps.com has Peter Laidler, Enfield Armorer par excellence where you can learn about this rifle and the Enfield subgroup has a lot of information about this grand old rifle. Alternately, try Stratton's or Skennerton's book etc. The short of it is like Malamute said, the older Sht LE aka SMLE aka No. 1, Mk. 3, simply had variance in the bolt heads when manufactured and the bolt head has to be fitted to the particular barrel/receiver/bolt combination. Later, this quirk became a bit more formalized with the 0, 1, 2, or even 3 in the later No. 4 series. These more standardized bolt heads made it easier to manufacture and later to do maintenance on the No. 4. to maintain proper headspace for the rifle (headspaces on the rim). These have to be fitted to the bolt but are easier to do so than the No. 1. Mk. 3. From what I have gathered, it is a fortunate accident that the differing bolt heads allowed the rifles to have their headspace adjusted by simply going to a larger number. But, different lengths exist depending on mfg. even on the numbered heads.
The bolt head on the No. 4, Mk. 1* can be locked into place--I once bought a barrelled receiver where the bolt was locked--it turned out to be the safety which can lock the bolt forward if the gearing mechanism is not timed correctly. Easy fix is the remove the safety and the bolt should come out--remember to be careful with the I* version as it has the slot in the receiver rail for removal where the bolt head has to be popped out (older version and the No. 4. Mk. 2 have a sliding catch that releases the bolt from the receiver just behind the receiver bridge). You can google it to reinstall the safety properly as it helps to have images of the procedure so you can visualize it.
It could also be where the cocking piece is failing to fully engage--if the weapon is not loaded--manually pull the cocking piece all the way to the back and then pointing the firearm at a safe area--dry fire it and operate the bolt as you usually do. The bolt should be able to be remove.
One last, not so good possibility that I can think of, is that the guy had a case head separation, did not realize it, and then crammed a loaded round into the chamber with the remaining part of the fired cartridge. That is really a gunsmith job as removal can be exciting and noisy if not done properly.
