Looks like a 1953 United more than a 1951. In 1951, United shuffle alleys still used score lights. They went to score reels in 1952. also http://www.pinrepair.com/bowl/cascade.jpg Not entirely sure about this but I think United made several different backglasses for the same game back in 1953. I had a 1953 United Chief and it looks exactly like yours---except for the backglass.