Though I have no actual way to be 100% sure, I think I have one too. I found an ancient countertop in the woods, possibly dating from the late 1700's, early 1800's. I cut it in half, to 2 21"x18" pieces. It is about an inch thick. I use
one in the kitchen oven and one in my LBE.
Like yours, it holds heat unbelievably long once hot, and does a great job.
And I must caution anyone else against tempting fate as I did. Putting an unknown material under high thermal stress could have been an explosive situation.
That said, I highly recommend soapstone. If you are lucky enough to find some, use it!