I'm afraid you're going to hate me, but here goes..........
Where the glass hits the metal there is a cup/dish shape holder. I would expect this to be approximately the shape of the glass when it sits in the metal frame - however it is noticeably larger than the glass.
Now, looking at the base of the glass, why would you have a knop here that is effectively hidden within the 'cup' that it sits in? Is there a locator in the middle of that cup? If so, then perhaps.
To my mind I think that the glass used to have a foot that got broken off. Then, at some stage someone had the bright idea of placing it into this frame. This is not an unusual thing to happen, whether it was during it's history, or somewhat more recent. Glass in frames often gets changed. In this case, to me, it looks good initially, then when thought through, I'm not so happy.
Still musing, Nigel