Yes, I've always found it a bit of a puzzle. It's generally only found on the later conversions - the earlier rifles seem (unless they've been FTR'd) to retain the surface finish with which they left their parent factory. For example, the BSA's are blued (right up to some 1944 rifles), Maltby's are generally (IIRC) oil blacked, Savage's retain their finish of, is it, 'dulite'? Yet, the later rifles seem to be brunofixed, as Peter says, presumably instituted at H&H's......