The only mention of a slash in front of a classifier name in the UML2 spec concerns interfaces of components. In ch. 8.3.1, p. 155:
Interfaces that are exposed by a Component and notated on a diagram, either directly or through a port definition, may be inherited from a supertype component. These interfaces are indicated on the diagram by preceding the name of the interface by a forward slash.
This seems unlikely to be the case in your example however. So my best guess would be that it's some non-normative use. Apparently some people use a slash to indicate the classifier to be an interface. But again, that's non-normative - the correct presentation would be using the «interface» keyword.