Rusty Iron Knot

Whereas the Iron Knot itself is traditionally the symbol of Muzan, the star who delivered magic, and is a figurative name for the two squares that form an eight pointed star, a literal iron knot that has been allowed to rust secretly extends this purpose. The original symbol of the Seventh star is well known enough that anyone who boasts it is bound to attract negative attention due to the connotation of magic, however rarely does anyone bat an eye at a piece of misshapen metal chain.

Significance

This item, when placed somewhere visible but not in obvious sight, signifies that the place is accepting of sorcerers. Showing that the owner would provide safe haven to any that cross into their path. Examples of how one would use it could be a merchant keeping it as a paperweight in plain sight, or perhaps it could be used tie a rope, or simply just among junk on a shelf. Therefore, any who have the gift or curse of magic know that they have support, and aid should they need it.
While rather simple to make, the symbol in itself is not something that someone with no knowledge of what it means would keep a hold of. Thus, the only holders are those people who truly mean to imply what it actually does. And because of that, and the fact that compassion for sorcery is a very unlikely opinion to hold, the item is very rarely used.

Origin

It is likely that the icon began to be used just by sorcerers who wished to be able to identify one another, which then naturally expanded to their friends and allies and descendants thereof. In fact, acceptance of sorcerers often ends up passing down from generation to the next due to parents teaching the same values to their children, and thus a rusty iron knot could end up being an heirloom, with it's aging look serving to prove the extensive loyalty a family has shown to magicians.