Sounds like it uses TrackPoint II, which was an earlier iteration of the TrackPoint as we know it. I have the first laptop I ever used, a Thinkpad 350C and it has this earlier iteration. The feel and caps used were very different.

The current version is the TrackPoint IV.

More on the topic, unfortunately does not include usage in external keyboards: http://www.thinkwiki.org/wiki/TrackPoint