Back on topic: With reference to the Native USB TX/RX LEDs ('pins' 73 and 72) - I just discovered that these LEDs use negative logic. That is, they light with digitalWrite(x,LOW) and extinguish with digitalWrite(x,HIGH) (the opposite to the L LED on pin 13 which uses positive logic).
So if it's not too much trouble it might be worth noting on the diagram that the LEDs on 72 and 73 are using negative logic. I was using them for debugging just now, and lost about half an hour wondering why my working code was registering a fault and my non-working code wasn't! 