There's no single, definitive picture, as its exact appearance (size of the U, bolt pattern, etc.) might vary slightly depending on the specific transmission and rear axle configuration in that particular truck. However, it will be a relatively robust, heavy-duty forged steel piece. It's not ornate or particularly detailed; its function is strength and reliable connection.
To find a visual, searching online image searches for "1955 Chevrolet truck driveshaft yoke" or "1955 Chevy half ton driveshaft yoke" along with terms like "transmission yoke" or "rear end yoke" might yield results showing similar components from that era. You'll likely see variations, but the overall shape and general function will be consistent.