The .303 cartridge in the attached picture came with the following description:
“These came from a military issue box with ‘MK 7 [broad arrow] I.S.A.A.’ Handwritten in red on the outside of the box is ‘C.S.P.’. “
The cartridges each have a short red slash on the case in slightly varying positions. Headstamp is RG 55 7. Primer annulus is purple, bullet has a GM jacket and is non-magnetic.
That is all the info I have. Can anyone shed any light on it?