I just noticed something in the fourth-draft script of ESB that becomes rather interesting when cross-checked against JW Rinzler's Making of ESB book.
During Luke and Vader's lightsaber duel in Cloud City, we get this exchange:
The wind stops and the fighting becomes even more intense. The deadly buzz of the laser swords reverberates through the vast power shaft. An alarm sounds and the public address system inside the control room crackles.
ADDRESS SYSTEM
Fugitives heading for transport Platform Southwest-One. Secure all transport.
VADER
Your friends may escape, but you are doomed.
LUKE
Not yet.
Luke slashes at Vader again, renewing the fight. Luke’s sword whistles past Vader and the young warrior is thrown off balance, his guard down. Vader’s light saber flashes out with deadly skill and cuts Luke, almost forcing him over the edge. He can barely stand. He wipes the tears from his eyes, but still can barely focus on his massive opponent.
The scene then cuts to Leia, Lando, and the others trying to unlock the door to the Millennium Falcon's landing pad. After Artoo gets jolted by the power terminal, we cut back to Luke's duel with Vader.
Luke is still wielding his lightsaber, but he finally stops swinging out of sheer exhaustion. This is the point at which the script calls for dialogue Insert B, featuring the revelation that Vader is Luke's father.
However, there's no mention of Luke losing a hand in this entire scene. And the special dialogue page, Insert A, which would presumably mention it, is not called for anywhere. So we can assume that, for whatever reason, this idea was temporarily dropped from the fourth draft script.
But we do know what Insert A said in the third draft, because JW Rinzler quotes it in The Making of ESB:
Vader’s light saber flashes out with deadly skill and cuts Luke's arm off at the elbow! Luke's forearm flies away in the wind as the boy himself almost goes over the edge. He can barely stand. He wipes the tears and blood from his eyes, but can still barely focus on his massive opponent.
The implication of this dialogue's apparent place in the script is that Luke's loss of his left arm at the elbow was not the end of the duel. Luke still had his lightsaber in his right hand, and kept fighting for a while until he was simply too exhausted to continue.
In the final scene of the third-draft script, Luke gets a new left forearm that has "metal struts and electronic circuits similar to Threepio's." Apparently this is the precursor to Anakin's droid-like prosthesis in the prequels.
Interesting that Luke should retain his father's sword while still losing an arm. I guess Lucas ultimately decided that Vader should cut away Anakin's sword as well as Luke's right hand (as opposed to his left arm), to heighten the tension (and the symbolism) further.
I also suspect that Lucas originally intended to have an echo of this early conception of the ESB duel in Willow--with Madmartigan losing his left arm to King Kael in the final battle, but still managing to kill Kael with the sword in his right hand.