That doesn't address the bit that has always intrigued me - the apparent blending of colors in the textured area. There seems to be a texture that goes with that, and an independent(?) texture made of all the diagonal straight lines. I was seeing some of it wrong before the add'l photos - I thought I was seeing white where it was just a reflection, so it's quite possible I'm seeing it wrong now, too. Perhaps the little diagonal "gouges" brought white up to the surface, which was then tooled with a dark yellow layer? Or frits were marvered on, fused then tooled?
Galle and Daum used an amazing array of surface decorations...I'd be looking toward French, myself.