I wouldn't risk it.
I am pretty sure that the Aussie made Sterns are toughened glass, only because I have seen a Cosmic Princess shatter into a million glass cubes when hit on the edge on concrete.
I think that maybe the older Gottlieb's are ordinary glass but I am not sure.
Toughened glass can take a fair bit of pressure on the face but hit it the wrong way on the edge and its game over.
+1. I have broken two backglasses, toughened glass both of them, they shattered into millions of pieces. One of them was an as new condition 6million$man backglass that i scored for $20 many years ago. Had it sitting against a wall beside a machine, lifted the pf of the machine, the ball fell out and clipped it on the edge 
OUCH! Don't you hate it when you do silly things like that, you kick yourself for days after
