That's a Brevex with an after-market square bridge. M400 signifies a Brevex with the .416-size bolt face - one of the better actions for .505-sized cartridges.

Carl Labuschagne is a well-known PH here in South Africa, now retired. He dabbled in riflesmithing and used a .460 A-Square of his own building for the latter part of his career. It had/has a Scout-style Leupold scope mounted as well. There's a photo of it in Pierre van der Walt's book. Carl was perhaps best known for his use of a H&H Royal in .577 NE for a major part of his career.

Armin Winkler is a Ferlach-trained engraver based in Paarl near Cape Town. He does fantastic engraving to this day.

That rifle did the rounds here in SA for a while. Last time I saw it, it had a Griffin & Howe-type sidemount on it. In fact, a guy I know still has the top part of the particular mount in his gunroom.

If memory serves, the rifle was exported to the US from a Pretoria gunshop in the early 2000's. No idea about the Bundock bit, unfortunately.