Grey provides the 'real' story on the Berwick discovery? I had assumed that the posters were found as insulation (glued in small stacks at Matt outlined) - but he says here that he believes they were initially used as signage glued together on a board. I guess it is still possible the boards were placed with the posters attached as insulation...
From the most recent Intelligent Collector and speaking about the Cimarron one sheet specifically:
This gorgeous poster from one of the first Academy Award-winning films was found in a small home, glued to a board with other 1930s-era posters. The boards had been exhibited outside of a theater, we believe, in freestanding sandwich-board fashion, with one poster glued on top of another as films came and went. The boards were sold at a small community auction and made their way to us. When I received them, I could tell they were a treasure trove and began steaming each poster off one by one. Many were classic titles, with some never seen in modern times. This poster sold for $101,575 in March 2012.