The posters are window card size and are of two styles. One shows Houdini coming to Providence the week of September 20, but this actually didn't happen. Houdini remained in Boston for a third week and his Providence engagement was delayed until the week of October 4, which is what we see on the second style card. (This is a key historical detail that, IMO, confirms these as 100% authentic.)
As wild as this find seems, it has happened before. In 2006 a similar discovery was made in the walls of a home in Chicago and featured on the show The History Detectives. A window card similar to these sold at auction in 2019 for $2,200.
The lucky homeowner is weighing her options as to what to do with these, but she is open to private sales. If interested, she can be contacted HERE.