The only dual marked example we have ever seen. We have never heard of a Disston & Son No. 90! The blade is etched Son and the eagle medallion has no son. Hamilton Disston (son) joined the firm in 1865 after his Civil War service. The eagle medallions were discontinued in 1865. The blade is crisp, straight and true. 5 point rip on one side and 8 point crosscut on the other. 26 inches long.