A hanko, is also called an inkan and is a personal seal. This seal is used to sign formal and legal documents in conjunction with a signature. Without this seal, business in Japan would stagnate. The hanko is necessary for filling in application forms or banking slips, or for when you receive registered mail. There are different kinds of hanko: cheap ready made ones — sanmon ban; ones that are officially registered — jitsu in; and those that are for banking mainly — ginkô in.
Mikao Usui would have used his hanko on his documents. The use of hanko will help modern researchers verify the authenticity of any documents that lay claim to Mikao Usui as their writer.