Here's a late 1930s photo of the Hoosier Hot Shots. If you look close you'll see Hezzie is posed with slide Whistle in hand. I included a slide whistle on my washboard because my goal was to have as many different kinds of sounds available on my washboard (within reason) as a possible.

In the various jugbands that I've played with over the years I always wanted to have the opportunity to include the slide whistle in more of our arrangements but differences among the members as to the creative direction of the bands never allowed this inclusion. These days I don't play much with my ol' Maytag. It's still a work in progress and I have plans to expand it and add more to it.