Bellow pics suggest a proprietary mounting boss that screws into the old brake boss hole. It makes sense that they would have incorporated the female spherical cutout in the ali forged body.
As an alternative, two pairs of spherical seat washers (one above and one bellow the bracket) could be used; this would work with the slotted hole! Only drawback is the £6 delivery takes the price for the pair to over £10 (obviously not such a big deal if someone makes a bunch of brackets up and spreads the cost).
Looks like all the pieces of the puzzle are in?
As an alternative, two pairs of spherical seat washers (one above and one bellow the bracket) could be used; this would work with the slotted hole! Only drawback is the £6 delivery takes the price for the pair to over £10 (obviously not such a big deal if someone makes a bunch of brackets up and spreads the cost).
Looks like all the pieces of the puzzle are in?