Thanks Peter, I value your advice. Just to be 100% sure before I pass on this rifle I've attached photos of the things I'm talking about so it's crystal clear.
The Z is stamped into the receiver and not present anywhere else, it definitely does not appear to be part of a 'ZF' or any other acronym.
The paint doesn't appear to be part of any lettering at all, it just looks like a dab on the receiver ring.
I recall reading somewhere else on this forum that a splotch of paint like this in Britishservice was used as an indicator that something hadn't yet been proofed?
If it's worth anything there is the typical 'NP 303 22.2, 18.5 tons' proof stamping at the muzzle indicating it passed a proofing round before being exported.
If you say I should pass I will, I just want to be 100% sure. Because outside of these two things it is a really good example of a wartime production No.4MkI that appears in very good nick.