Vette, the 100 uses a different arrangement than the 6-cylinder cars. The shaft extension is connected to the distributor with a roll pin, in place of the offset drive dog used on the 6-cylinder distributor.
Catfood, before you remove the extension, scribe a reference of some sort on it to allow putting it back exactly the same way. As the gear has 13 teeth, it needs to be properly reference to the upper distributor components. My suggestion would be to use the rotor as your reference.