TO-220 voltage regulator, heatsink and mounts
Here is a site that shows the uninitiated how to mount components on heat-sinks (amongst other things). Towards the end they touch-on mylar insulating washers (like the one you have). For a simple regulator you don't need this insulating plate providing you make sure the heatsink doesn't touch parts of the circuit that are not at 0v (centre pin on most regulators)