Is it a 261/1 or 265 pump? If so, those pumps have a brass goodie over the TA adjustment screw to keep it from being tampered with. This was for smog-compliance reasons. The AROC Spica notes say you can pierce and remove the plug, and the screw is underneath.
This is akin to the plastic stops on 70s US cars carb idle circuit screws that kept you from adjusting it more rich than was allowed.
I have a 261/1 pump (77-79 Calif Spider) that has this plug. The plunger should move up and down when you push the throttle bellcrank toward the reference screw. If it doesn't move, it's stuck, but I've never seen one that way.
Is this on a Berlina? Then didn't come with pumps this late. 255/1 would be the last pump tipo on a Berlina.