Weclome
I may one day make this my desktop background.
Looks like slate. The last slate panels I dealt with were face plate differential regulators in a paper mill in Grisby, east coast of England.
A little tale - a digression but mods, be kind.
We were modifying it but the mill shut down as have many others here. Anyway, the particular machine was bought by a paper making company in Taiwan. The regulators were in the field control of DC motors so had to deal with the field current. We had redesigned the system to provide PWM control of the fields. This meant that the face plate resistors could be much reduced in size.
That got done in Taiwan. But they didn't have slate so the regulators got rebuilt on marble - looked the real deal.............