There's 3 types of brake mount. Sunday have their own so it won't be them. The "Neal Wood" mounts are probably the most common, these have one long bolt that goes through the brake, through the mount and into the frame, holding the brake onto the mount. The mount has a curve on the bottom so it fits flush with the frame when tightened. These are the better type that are less likely to come loose through normal use.
The other mounts thread directly into the frame, then a bolt holds the brake onto the mount. These are more likely to come loose as when you tension the springs you need to loosen off the bolt, which will loosen the mount too if it's not securely tightened.
Just had a quick Google search, it looks like that frame uses the latter. Is the top of the lug flat? Or does it curve with the seat stay? If it's flat it's the latter, if it curves it's the Neal Wood