The documentation for writing Registry Plugins says that the Plugin Type is derivable from the Pluggable Model via "underscore" inflection, and specifically uses the example of ExternalIdentitySource plugins being of type external_identity_source. However, later on the EIS plugin type is defined as source.

Review the code to see if this automatic inflection is actually used anywhere. If so, then source plugins need to become external_identity_source plugins. If not, the documentation should be revised.