The Quackenbush version may be highly collectable in the UK but this type of gun generates about zero interest in the US.

Is it possible that the Briton and similar were all produced by the same third party? That is, manufacturer and retailer were different companies. That would go to explain the lack of markings on the boxes. If so, then the guns should all be exactly the same dimensions within tolerances.

It's hard to nearly impossible to produce two sets of production tools that turn out exactly the same press metal item. There is always some small indifferent variations between two sets of tools.

This would also go to explain why the manufacturer has been hard to determine; the "XYZ pressed steel manufacturer" would be a needle in a haystack. It could be a single guy in his basement churning out pressed steel guns and sending them off to his big customers.