I think it's perfectly reasonable to suspect that 3PO had some hardware upgrades in between the films. Take into account how old he really is and how many times the poor fellow has been blown to pieces. Also, I think that the profile on Vader at Wookiepedia references him making upgrades to his saber hilt over time.

From the Wookiepedia profile on Anakin Skywalker:

"After suffering his grievous wounds on Mustafar, Vader created a new lightsaber. Vader apparently modified this lightsaber over time."