Yucca Surf Mat - Regular swim which you can tie up

which you can tie up at your preferred length

You can see the major influence that Transition Era boards of 1966/67 had on Kegel with this model

but is rather flat

From the shaper himself:

Yucca Surf Mat - Regular swim which you can tie upThe Merlyn surf mat designed to give new riders the smoothest possible introduction to mat riding. Built with slightly increased internal volume, the Merlyn provides extra lift and stabilityhelping new mat riders find trim, control speed, and connect with the wave more easily. To make tuning your setup simple, the Merlyn features a Boston valve for quick, user friendly inflation and precise air pressure adjustments. This lets riders fine tune the mats