Blacktail, the bow on pg 68 doesn't have a description but it looks like a simple D bow painted with white dots on a black background (on the back of the bow).
As far as getting a 20" draw from a 50" long, unbacked bow, it shouldn't be a problem with white oak...unless the bow is narrower than 1-1/8" and pulls more than 50#. When you start getting into the 50+ poundages, you need to make the bow wider. And yes, 50" is within the range of the bow lengths of the Apache bows I've seen, especially with the Kiowa Apaches.