I am confused and maybe someone can help.

I purchased what I thought was a 520/7 Ideal. In with the package are some plate holders and one contains an old plate. But the glass size is 3.921" x 0.052" x 5.854", 99.6mm x 1.32mm x 148.6mm which seems not to match any imperial or metric published plate size. I realise that the plate purchase size is the image size, but cannot see how this size fits with standard sizes? Was this a special manufacturers plate?

Russ