I'd say what you've got here is the by-product of poor stock blank selection by the manufacturer. IMHO, you're looking at a piece of sapwood with only a small amount of heartwood running through it due to the way it's been cut. Often, these cheaper ends would be supplied steamed to darken the sapwood but I'm guessing that's not been the carried out either, so over time you're getting more and more visible discrepancy between the two
whilst I mean no disrespect, this is a very low end bit of walnut which if for an air rifle would probably be acceptable, but if for a RF or CF would be a considerable no-no.
Get onto Hull and have a go, about bloody time they pulled their fingers out.
You are fortunate that you got a reply from Hull Cartridge Company, they have never replied to my enquiries about rifles that they are supposed to be the importers of. Never mind it's their loss because I took my money elsewhere and bought another make. I understood that they were supposed to be a good company but not in my experience so can't see me buying any of their products at any time in the future.