This tool is a Vertical Lined Thumb Print Stamp and is number P861. The tool measures 5/8" in length and 5/32" in width. Stamped on this tool is "P861" and "Craft Japan". 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.