You pose an interesting question.

Stickers - can be removed, or pasted over.
Colored bands, cut off.

My guess is, you are looking for a unique identifier that can identify your property, in a police photograph of 20 almost identical instruments?

"Chipping" as in an animal IS is the most obvious. However, I believe you would like a remote ID, from a remote source. Using an "aPP ID" with imprinted pattern on the outfacing bellows may be discrete enough to flip the fellons. You may select a color frequency that is scarcely disernable to the naked eye, however leaps out when viewed in a pile on police contraband.

Sincerely,
gargoyle

. Insurance is cheap...there is not a hot market for stolen concertinas