Remember something about the guys taking a small hack saw and cutting a notch in the base by the plug where the mixture adj screw is hidden. Then they would take a small punch to remove the plug to expose the adjustment needle. Its a tamper proof thing that Ford did to try to make that horrible emission engine comply. Don't get your fur up, the engine is great, just not a green engine, just like the 360/390 engine family, not very fuel efficient.
Good old Bill