not sure if this helps, but worth knowing, if you get someone to take photos of your work, unless you specify, and maybe even pay for it,
the copyright lies with the photographer. you need to get a release from him/ them.
in the same way, if someone designs a web site for you, you may have to pay extra to retain the copyright to the site yourself.
as for copyright, what you are doing by adding the mark is suggesting that you have imparted a specific "extra" to the item.
experience suggests that you can spend some money down at the local county court to sue someone who has copied directly your item, but frankly the biggest problem is about the life of copyright.
i also design models, and we work from original works drawings, and occassionally we buy those from one or other museum. some seem to think they have copyright, others don't. in my opinion unless the providers of the item are direct descendants of the originator they would have a difficult job proving infringement of copyright.
last thought, years ago, an american company selling chewing tobacco sponsored some racing cars. once models were made, they went round to the retailers and threatened them with the law. some gave in. others said how can you enforce a copyright on an item which is already copyrighted.
i.e. if the car was a porsche which is a copyright design, how can the advertising be copyrighted too. the other point is why would you advertise, and then want to stop people buying your advertising paraphanelia????
murricans i guess.
anyway the normal advice is put the copyright logo on, and the date and your name, then wait and see what happens. if nothing else you may get some extra advertising.
paul