[Arran]

Bob;
Those black stains are likely iron stains, rather then ink, they seem to show up on walnut, solid or veneer, caused by water from a potted plant, vase, or who knows? I've heard of some using wood bleach to fade them but I found that phosphoric acid may also work, wood bleach is often oxalic acid anyhow. Regarding that split in the base near the edge of the speaker opening, would it be possible to put a dowel through it? I was going to also suggest using a wood biscuit but that board would have to be split apart to do that.
This style of "cabinet" must have been fairly popular among A.K models as they seem to come up more often then other alternative floor model cabinets, other then maybe a Philco 70/71 chairside, I don't know about the Brand Z models but they tend to be somewhat rare among most makes. There is a CGE one that seems to pop up in my area from time to time, but the chassis they chose for it was a bit of a letdown, one of those typical 1937-38 models pulling the 6H6 + 6F5/6J5 tube stiffing "scam" rather then using a triode-double diode, and it has the wrong knobs on it.
Regards
Arran