K5 is Kynoch (I.C.I.), Kidderminster, UK (1944)
I suspect .303 British. The Brits used things like 11Z to indicate component loads.
(VII Z on .303 British indicates Mark VII ball ammunition loaded with nitrocellulose powder.)
The 44 indicates made in 1944.
A lot of milsurp ammo sticks to a magnet, even non-AP loads. Portugese 7.62 ball ammo sticks to a magnet.
A good place to check this stuff out is:
Headstamp Codes