G Mark I is Tracer Mark I
This stuff is manufactured by the Dominion Arsenal in 1929.
The black band is indicative of special lots issued to the air force, 'way back when. See the series of articles in "GUNS REVIEW" magazine by Peter Labbett, back about 1967, series called "The Banded .303s".
Banded .303s in Canadian manufacture are not common.
Canadian/British ammunition of that period indicated the various "Specials" (AP, API, APTI, T, Blank and so forth) by the colour code of the primer sealant and by a letter on the case-head. We did not start using colour-codes on the bullets until well into World War Two, picking this up from American practice.