A broken chain link

A single 'broken' chain link, seems meaningless to the average passerby but to fellow members of the rebellion it's a subtle sign of safe harbor. Designed to still be functional, like an open carabiner or a hook, the link can be incorporated into the chains from which a business' sign hangs, a discreet flag for those who know where to look. It can also be added to a ring of keys, hung from a belt, off a torch sconce, practically anywhere you may need to quietly share your affiliations.
More decorative versions can even be quietly incorporated into jewelry.
As far as anyone with the rebellion is aware their secret symbol has so far successfully remained as such, though as their activities become more overt and forceful and the empire's focus is drawn their way they know there is a risk it is eventually discovered. The hope is that the triviality of their chosen symbol will hide them long enough to succeed in their efforts.

Manufacturing process

The techniques are pretty standard, any smith worth your time can make a chain link or a hook. Where it gets interesting is combining the two successfully so the link holds.

Significance

Serves as a secret indication that an establishment or person is a member of the separatist movement that seeks to dissolve the empire. Their primary symbol is a pair of horns severing the chains of the empire, a take on the Keepers' symbol of chain bound hands. The single link serves as a secret handshake type symbol but also is symbolic of both how a chain is only as strong as its weakest link and how they as a person have already severed their bindings to the empire.

Item type

Miscellaneous

Rarity

Quite deliberately, extremely common

Weight

Varies

Dimensions

Varies

Base Price

Varies

Raw materials & Components

The base material can vary, most commonly it's a metal usually iron or steel, but more ornate versions have been made for people.

Tools

Standard smith's tools will do the job just fine.