This tool is a steel Geometric Box Stamp and is number O51. The tool measures 11/32" along each side. Stamped on this tool is "O51" and "Kyoshin Elle". This is a great stamp that produces a nice impression.
The growth and popularity of leather carving in the USA in the 1950's and 60's spread to other countries. While Craftool stamps were the standard, they were not commonly available to craftsmen in foreign countries. In Japan, a couple of companies started producing tools similar to Craftool but also became great innovators. Those companies, Craft and Co. and Kyoshin Elle, produce fine quality tools in unique designs along with the basics.