Canti boss diameter is just under 8mm, U-Brake boss dia is just under 9mm. The HS33 mountings are slotted to just over 8mm for canti bosses so they won't fit over the U-Brake bosses.
Must admit that I thought that they were the same diameter aswell, just different placement :?
No worries, it gives me something to do and means that the finished bike is a little more special to me 'cos I've had to do more than just 'bolt bits on'.