Hi Martin,
It's not "concave" or "hollow" that is the point. What is typical is the "very coggy" appearance of many of the canes, often with the same multi-cog pattern repeated a few times in the centre of a cane. As for the "cross" canes, they come in a variety of colours but always look very strong when set amongst regular millefiori canes - and as far as I know (although I have only seen a few examples) they have a distinct style with four "arms" (if that's the right word) and a contrasting colour central rod.
The first weight I saw of this type was sold to me by a person who uses this board, and I bought it partly for research to see if it could have been "Scottish" as was suggested by an earlier seller. I will add a photo soon.