How to mount a fresnel to the Sinar P2 8x10

[magicman]

Hi all,

really hope you can help me with this -
I just got a fresnel for my Sinar P2 8x10 with a metering back.

Since I´ve a fresnel for the 4x5 back, I was full of confidence to be able to mount the 8x10 fresnel as well - but I don´t.

As far as I see, the fresnel needs to be placed under the two latches (on the upper short side, when back is in portrait ortientation), and the fresnel should also slide into a slot on the opposite side.
But there is kind of a stonish material filling the slot, which seems to hold the ground glass in place, and which hinders the fresnel to slide just in. Please see the attached photo...

Is this normal, and do I need the remove this prior mounting the fresnel to the back?

Many thanks in advance for your answer.

All the best,
Mario