I just had this very problem on a F Grade. The gun was dry and dirty, not allowing the hook to seat fully in the action bottom. I cleaned it thoroughly in my ultrasonic cleaner and lubricated it.

As others have said, with the forend off, open the gun by pushing the breach end of the barrels toward the breach face. The clearance between the hook and barrel pin is tight.

If the barrels are allowed to slide ever so slightly toward the muzzle, the pin will engage the hook.

Hope this helps.

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