Full-bodied in both tone and stature, the aptly nicknamed "Jumbo King” is the largest size offering in the Waterloo line and a modern day tribute to jumbo-sized guitars of the early 1940s. Originally manufactured by Regal for the mail-order enterprise Montgomery Ward, this slope-shoulder body size was designed to provide performers of the era with enough volume and power for professional radio work. The WL-JK accomplishes this and more by employing a full five-inch body depth and a slightly narrow waist to enhance both the bass and treble response.

The WL-JK is available in a minimally appointed “standard” version as well as a “deluxe” version featuring added bindings, true-to-era "valve cover" tuners, and a full-body sunburst finish. Both models utilize vintage inspired x-bracing and prominent v-shaped necks with fully adjustable truss rods. These models are also available with optional East Indian rosewood back and sides, as well as an optional Aged Finish for an authentic time-worn appearance. Hardshell TKL case included.

All Waterloo models are handmade in Austin, Texas.