Hi R47,
The US Copyright Office do tend to be fairly strict in what they accept for registration. There's a simply enormous
Compendium of rules the staff have to follow during the examination process! There are a number of reasons why 'insufficient authorship' would have been cited. In all probability they may have thought it lacked sufficient originality to be registered.
Under UK law, the threshold for originality in artistic works is not that high as long as it is clear that a work is the product of a human mind and has not been copied from elsewhere. However the criteria for copyright are different to those for design right.
There is no official process for registering
copyright in the UK, and for that reason there is no 'Copyright Office' as such. Any body purporting to offer registration will be a commercial service, which despite their claims, is of pretty limited utility when it comes to knowing if a work is original enough to qualify for copyright protection. The UK Government department for overseeing copyright policy is the
Intellectual Property Office which also looks after patents, designs, and trade marks. Only these latter three subject areas involve a registration process, the most exacting of which applies to patents.
You mention the public domain. In theory, yes, your 'design' is now unprotected with regard to design right, but copyright still exists in the watch face if that can be described as an artistic work. That was why I previously suggested putting a copyright notice on your product packaging and advertising, in order to make it clear that you are asserting that intellectual property right, even though design right may have lapsed. Design right is intended to protect the appearance or shape and styling of a product, which is subtly different to the purpose of copyright protection.