I can't seem to find the Boeing Model 908-303 in Tony Buttler's American Secret Projects volume regarding fighter jets. It's quite apparent that the Boeing Model 908-303 documents came into light on the public domain online after Buttler's book went to press.
It's "quite apparent" you don't understand the purpose and scope of Tony Buttler's
Secret Projects books. They are not, and were never intended to be, a comprehensive account of every single aircraft design ever drawn. The primary focus is on final submissions to actual requirements, with some degree of attached seriousness, including desk models, windtunnel testing etc. There are thousands of studies which went no further than a drawing or two - these will rarely merit inclusion.