I'm guessing it's something a bicycle manufacturer made to get into the trail machine market. What makes me think this is, it uses a bicycle steering tube, the front forks resemble that of a bicycle of it's era, the fenders are supported with what appears to be bicycle supports. The way the top frame tubes are rolled inward towards the steering stem leads me to believe that it wasn't homemade along with the way the lower tube is tapered and closed leading to the rear axle hanger. These were all typical things bicycle manufacturers did making bicycles in the 50's. The engine plate also bends down in the front for added strength, not something the average shmo would do while building a homemade trail machine, to me it's just too cleanly done to be homemade.