The “E K” on the headstamp of the Greener Police cartridge is Eley-Kynock.
The various “calibres” for the Greener Gun, (a modified Martini Action)
used several brass cases, of which the recessed head 12/14 gauge Bottleneck case is the last in the series, and probably the most common.
The recess in the base was for the two side pins connected to the body of the firing pin, so as to prevent the use of Civilian 12 gauge cases in a Greener Gun (Colonial Gun Control).
The other two , less common shell variants were a 12 gauge case firing in a 14 gauge barrel ( tight choke, one would say); and the Bottlenecked 12/14 gauge case, which reduced the pressures somewhat; this latter case did not have the head recess.
The “Production” version of the gun actually had a "13 Gauge " Barrel ( Barrel is marked “13”); this is equivalent to a “Full Choke” setting on a normal shotgun.
E se Chiel le 'd urigine piemunteisa, cuma mai viv ent el Friuli? Magari se l’e mes friulan, se capis?
Salut, e cerea,ne.( Prov. 'd Cuni)