Not that difficult:
Let's say there are 4 contracts outstanding at any point in time (there are of course many more, but let's keep it simple).
EDH4, EDM4, EDU4 and EDZ4
We just established that on 1-1-2004, EDH4 = ED1, EDM4 = ED2, EDU4 = ED3 and EDZ4 = ED4
Fast forward to 1-4-2004, EDM4 is...