the glass liner makes perfect sense
The form of this cup supports the idea of glass liner.On the other hand, look at this Austrian cup and saucer:
viewtopic.php?f=24&t=29919No glass liner would fit securely in there because it`s perfect hemisphere, I tried - it tilts and falls off.
In a word, still a mystery to me; liner or no liner, at least in some cups. On the other hand, it`s unlikely tea, coffee or Vienna Milchkaffee (Café au lait) were served in silver.