Physical analogy to introduce a Delegate in .Net programming
I know that you are looking for a physical analogy, but I'd say I agree with @ctrl-alt-delor about the 'link to what they already know' part.

I usually start by introducing a simple algorithm, then show how that algorithm can be parameterized. Then we move on to OO, where I discuss how an algorithm can be parameterized with not just a value, but with a piece of code (i.e. creating an interface, and passing an implementation of that interface to a method). For the interface, I always use an analogy like USB.

Then we move on to the delegates, where I introduce delegates as if they were an interface with one method, Invoke(), and whatever you pass to the delegate gets called in that Invoke() method. Then we move on to delegate internals (how they are immutable, and multicast etc.) I also have examples and practices that we do to show them that every solution that uses a delegate can be translated into a more OO version using interfaces (again, referring back to interfaces).