Tim,
Yeah, the spine is Jimped in the curve. took a little playing with a file to get that to work.
M390 is a pretty high end Bohler Uddeholm micro particle gen 3 steel. I would say it is probably about equiv. to S90V as far as cutting, however it doesn't seem as difficult to sharpen, although I've sort of lost sight of which are hard to sharpen and which aren't . The heat treat is fairly high running up about 2150ish at peak with a couple of stress relieving stages. (Basically identical to elmax) Fairly involved temper process with 3 separate stages and multiple cryo steps involved.
Then end result appears to be rather incredible from what I can tell. I've found nothing that comes close to elmax for general purpose - "average guy" knife steel and from my reading M390 is probably a little to difficult to sharpen for the average guy, but I was surprised to see that this took an edge fairly easily. (Sampling of 1 of course) It has a ton of chromium in the content so it is extremely stain resistant. It isn't supposed to be quite as tough as Elmax, but will hold an edge a bit longer. We'll see how it behaves, but it sure does sharpen up to a razor's edge. It has that buttery feel that I like when sharpening that just felt exactly right. Now to see how it holds up.