From the Herbert H. Warrick, Jr. Museum of Communications, who note: "I've seen this style of dial illustrated in early human-factors study reports. I didn't know they actually made them!
I've seen this style of dial illustrated in early human-factors study reports. I didn't know they actually made them! pic.twitter.com/z1Z9CHhQOD
— Museum of Comm (@museumofcomm) March 19, 2017