Lister Noble
The primer lock is positive, the pin in the primer piston should be engaged into the slot in the primer outer case.
The position of this locking detent is indicated by a small hole in the primer outer case, the only problem with seeing this is that due to the way that the primer is fitted into the panel the hole might be at the bottom and not visable unless you lay on the floor and look up !.
When you push the primer piston and turn it to lock the piston so soon as the piston locking pin is engaged apply a LIGHT pressure on the piston towards you and rotate it , you can then feel it when it engages in the locking detent.