Many thanks to Anthony Shaffstall (the son of the inventor of this device) for sending in this 1960 advertising material.
Said Tony: “When I was a kid I accompanied my dad to many newspapers in the area where he would, late in the evening, install these devices on the Mergs. I was in grade school and didn’t understand what he did but I was mesmerised watching those machines run — still am.”
More details of this device can be found here: Shaffstall Mat Detector