That may be a hard question to answer. The .455 HE/2nd models were produced under a military contract for the benefit of Great Britain and, to a lesser degree, Canada. I am unaware of any strictly commercial production for this model, so explaining the one you are looking at might involve finding guns diverted from a military delivery or perhaps shifted from military to private hands without the benefit of the decommissioning process that involved stamping all those proof marks. I have seen references to other "commercial" units of this model, so they must exist. But I don't know if we are talking about a few dozen, a few hundred, or a few thousand out of some 70,000 manufactured.
Does this gun have a lanyard loop, or a plugged hole where a lanyard loop might once have been mounted?
Leaving aside the curious fact of the military markings (or lack of markings) on this gun, I might be tempted to spend about $700 for a 90+% .455/2nd that was still in its original configuration. If someone could persuade me that a commercial .455/2nd was a known collectible with some additional value, I might go higher. I doubt I would go over $850 on trust. Somebody would have to show me comparables.
If you get it, please post pictures.
