The very Edward Gorey-esque drawing was what drew me in, but after researching the inside of yesterday's advertisement, it would seem that the big draw was really the "spirit slates." In fact they are still fondly remembered (and sold) today.
Owen Magic describes them:
These slates were originally designed by Carl Owen and are positively the most ingenious slates for producing two sets of writing ever designed. The slates may be freely handled by anyone and, with nothing added or taken away will apparently produce writing on both blank slates when they are held together by a member of the audience. This is without a doubt the nearest approach to genuine spirit writing ever accomplished.The video below is someone giving away the secret of using Spirit Slates.
Spirit Slates Magic Trick Instructional Video - The most popular videos are a click away
I was also able to link the inventor, Carl Owen, to Thayer via OwenMagic.com's "history" page:
The more I read about the men from this period, the more I learn how interconnected they were with each other.
Owen Magic Supreme can trace its roots to a June 1891 advertisement in Mahatma magazine by Floyd Thayer. In the August 1902 edition Floyd placed another ad, offering for sale the Wand of the West.
During the fall of 1913 Carl Owen entered the firm, and was later joined by his brother Emmett. When Floyd Thayer divided the firm in 1933 the Owen brothers purchased Thayer Manufacturing and absorbed other companies, the better known of these being Merv Taylor Magic.