It's confusing. Even the manufacturer gets confused. If you go to the two links in post #71 and open the links to Product Data Sheet you will find that on page two of each (top right hand column) that the same molecular formula is given for both compounds. One of them is wrong, of course.

I'm thinking of trying to ascertain which of the metaborates I have. I note that their solubilities in water differ, so I'll try to dissolve the one that's most soluble at 20degC and if it doesn't all dissolve I'll add water to see if its solubility matches that of the other one.

But tell me, I'm used to solubilities expressed as g/L of solution (or 100mL of solution). The product data sheets show solubilities as "percent by weight." So does this mean that x grams dissolves in 100mL of water to make an amount of solution that's probably not 100mL?