When did slide outs first show up?

Thanks everyone, just to get a visual of the technology, were the tip outs on a piano type hinge on the bottom of the tip out and simply "tipped out" when the trailer arrived at its destination?

The Canvas covers for the open areas while traveling could cause some pretty serious issues. But I would imagine that these destination trailers pretty much stayed put when they got where they were going.

thanks!

jeremiah