The difference between an unregistered bearer bond and other types of bonds is that the bearer bond is not registered to a particular individual, but instead guarantees payment for the bearer of the bond. Other bonds are registered with the social security number of the bond recipient.

However all bonds have serial numbers, regardless of whether they are registered. This is how the government verifies the validity of the outstanding debt guaranteed by the bond. Otherwise, what is to stop some counterfeiter from manufacturing fake bonds and claiming them?