I don't know how to lock it , maybe be as easy as tightening a screw that I believe is under the rotor , but those old mopar race units are not adjustable like the current MP electronics distributor ...

I'll refer to Rapid Robert on how to adjust the mechanical advance on the old units , involves welding and filing , I'm sure he has his procedure on a clipboard on his computer for a quickie copy and paste because it posts it up a few times per week ...