In Phantom Menace, after R2 repairs Amidala's ship and gets everybody safely through the blockade, they're brought in front of the queen to get a commendation.
From the script:
QUI-GON, OBI-WAN, CAPTAIN PANAKA, and the LITTLE BLUE DROID stand before QUEEN AMIDALA and her THREE HANDMAIDENS, PADME, EIRTAE and RABE.
CAPT. PANAKA : ...An extremely well put together little droid. Without a doubt, it saved the ship, as well as our lives.
AMIDALA : It is to be commended...what is its number?
The LITTLE BLUE DROID lets out a series of bleeps.CAPTAIN PANAKA leans over and scrapes some dirt off of the side of the DROID and reads the number:
CAPT. PANAKA : R2-D2, Your Highness.
But in the next scene we get a close-up of the same side of R2 and no indication of their number is visible.
So how did Panaka know R2's serial number? Is there something he did (besides wipe away some dirt) to reveal it?