Without seeing the item a positive ID is unlikely. Taking the marks out of given order:
12HC - This ought to be a shotgun 12 bore but is probably something else.
'Arrow' - the standard War Department marking.
/40 - Year of manufacture (1940) of the projectile?
761A - ?
CY12/42 - Propellent filled by Chorley Royal Ordnance Factory, UK, Lot Number 12 of 1942