When the item matters more than the owner: a code of its own, with photographs, a description and the history you can show for it. Tag a guitar, a bike, a lens, a tool that walks off job sites.
Your phone number identifies you. This identifies the thing — which is what you want when the thing changes hands, or when you own forty of them.
Nine characters with a check digit. A wrong character is rejected outright rather than resolving to somebody else's item, and O/0 and I/1 are corrected for you.
Serial plates, scratches, the mark that makes it yours and not one just like it. What you would otherwise be describing from memory to an insurer.
Where it came from, when you got it, what has been done to it. Dated as it is entered, so it reads as a history rather than a claim written afterwards.
Per item, and one account holds as many as you need.
You get a short code. Nobody picks their own — they are issued at random, so there is no scramble for good ones and nothing to squat.
Engrave it, label it, hide it inside the case. Print the QR for the ones with room.
A finder, a buyer, or a pawn shop can look it up and reach you without your number ever being on the object.
You choose per record, and you can change it after something goes missing.
The page shows what you put on it — a name, an email, a phone, whatever you want a finder to see. Nothing is shown that you did not type.
The page shows a message form and nothing else. Not your name, not your number, not whether the record even belongs to anybody.
A finder's message reaches you. You decide whether to reply, and whether to reply by email or text. You can offer a reward. They never learn who you are unless you tell them.
Things worth knowing before you put this on something that matters.
A number is a lease, not a property. Phone numbers get recycled. Whoever can prove control of a number by SMS holds the slug — documents do not override a failed proof, because that is the only rule that cannot be socially engineered.
Nothing resolves until it is proven. An unverified claim is a private draft that expires. Typing a stranger's phone number in gets you the same response as typing one that was never registered, which is the point.