Attend ONE marble show and they won't be mysteries any longer. You can learn in 2 days in person what will take you 1.5-2 years struggling to learn from flat, one-dimensional photos of varying lighting, focus and color balance on the Web.
They will need to be a person very well versed with contemporaries, as well as some good basis in vintage handmades and some minimal grasp of Jabos/DAS etc.
There aren't that many of those people about.
Note the wide black band edging. I think that is what you are referring to as it narrows.
An Akro employee told us that they used to throw bits of metal scrap into the pot just to watch it flare up. Amusement. I think pretty much of it all burns off.
And that confidence is based upon?
I expect that in quoting specific values to the OP sight-unseen, you are ready to offer those amounts that you are confident in.
Good, well-focused pics. Newer (and some established!) collectors would do well to look at the glass and it's motion. Its very illustrative of this type.
The only way for anyone to move that footlocker would been to scoop bowls of marbles into buckets to empty it. Six people wouldn't have been able to get it out of the van it was in. He must of filled it the same way. The wood footlocker wouldn't have held the weight of moving it in any case.