The Adventure of Channel Marsh by Arthur Morrison

Mayes never recovered consciousness, and was dead when we visited the hospital the day after; both skull and spine were badly fractured. And the very last we saw of the Red Triangle was the implement with which it had been impressed, which was found in his pocket.

It was a small triangular prism of what I believe is called soapstone. It was perhaps four inches long, and the face at the end corresponded with the mark that Hewitt had seen on the forehead of Mr. Jacob Mason. It fitted closely in a leather case, in the end of which was a small, square metal box full of the red, greasy pigment with which the mark had been impressed.