The original Signature (dubbed Signature I in sakwiki.com) is the precursor to the current Signature with retractable ball point pen (usually blue ink). The original Signature model has the removable ball point pen like what the 91mm with plus scales model have.
I'm still researching when exactly did the change between Signature I and II happened but it is sometime in the 1990s. By 2001, the Signature model already features a retractable pen.
There's also another model called the Stylus which has a touch screen stylus instead of a retractable pen.