its my understanding that there were 3 different caliber for original greener police guns: 12 ga with 2,5"/65 mm long case

14 ga to make it harder for native freedom figther/terrorist to get ammo for the gun
I know this very well because had once a lot of this 14 ga brass I changed for an early black powder double gun. the rim was to thick to get it into the camber so I turn it down to 14 ga CIP measurments

the 12/14 special case with the groove in the base

because this gun here is proofed for the 2 3/4" /70 mm long case I thought this must be a factory made sporting rifle.