Yeah true, I think I recall it being some form of a household insignia where it denotes her already belonging to someone else's family so she won't be harassed by any idiotic slave trader that wants to traffic her into the business....
Makes sense, yet you would assume that well dressed women that walk around Zeta would be presumed to be citizen and of a decent position. Clearly slave dealers wouldn't caputre them within the city limits and outside... probably the collar wouldn't make much differenct.
Now, metal slave collars, as something you wear everyday, not just a constraint for transportation, i don't know if they happened anywhere else in hystory but they were a costume of roman society. They where just metal rings the slave couldn't easely remove without tools and expertised with the name of the household to prevent easy flights, since family slaves in Roma pretty much roamed the urbe for all kind of errands and where rather trusted, but they had to be told apart from citizens and they could still escape.
I think the brilliant idea behind slavery in Roma was that a slave had all the right to buy off his freedom. So slavery could have been seen more of an unfortunate temporary circumstance by a slave, who was motivated to work well and faithfully for his owner, so he could become a liberto quick and still work for the household as a free man. To be a liberto of an influential family was still a position of prestige.
Some of them. Of course you had slaves dieing in mines and fields and being abused too.