I need to change a full house of pvc window hinges. They have go 4 screws on either side of the hinge. Two Philip's which I can take out but then the hinge has 2 screws in which have a circular head which I cannot remove. These are not rivots as you need to take these out to get the hinge off and they do not look like allen key and are not torx. Anyone know how to remove please ?