larry Brown is right on with the period of proof. The 1 1/8 is the mark applied to indicate it has standard 2 1/2" chambers. If it had 2 3/4" chambers it would have the 1 1/4 oz. mark. The chamber may measure slightly longer due to the angle of the forcing cones or, if someone has lengthened them which would render them out of proof under British proof law. I think that you will have a standard 2 1/2 " chambered gun. Just an adition; if it was meant for 3" cartridges it would have an 'L' after the 12 over C in the diamond to indicate 'Long' chambers or the 1 1/2 oz. mark. Lagopus.....