.303 with Red Mark on Case

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?



Unsure what CSP stands for but that is a standard mk 7 ball round produced by Radway Green in the UK. Given the age and the fact that the original neck and primer crimps are intact, I guess that it has been marked as surplus ammunition. I’ve got a box of surplus, repackaged Winchester WWII 303 ammunition, which was sold for sporting use. Radway Green ceased production of 303 in 1973. There is a lot of surplus 303 floating around. I’m currently shooting my way through a crate of 1966 POF Mk 7 ball.

I don’t know what C.S.P. means either, but with it being handwritten on the box & each round marked it could be a shooter with the initials CSP so if he / she sets the box down it will be marked and the rounds when fired would be marked and they can be recovered for re-use. A common practice for match shooters. However re-loading Berdan primed brass is not something everyone attempts.
That shoots a hole in that theory, but still?