If you can't scratch it, it is enamel. Essentially glass! Line printing you tend to mix the colours you want and then print once per colour. Versace, and similar, scarves have over 40 separate colour printings but they ain't glass. The advantage over screen colour printing is you get the exact colours you want. Obviously with silk screen you are unlimited in colours and nuber of printings... apart from cost and space of course.
I would guess these were probably produced in the hundreds, particularly if selling dept stores. Hope you can track the artist, might lead to the glassworks or at least country of production.