Some notes on Luke's severed hand--perhaps useful for updating the Secret History article.
(I sent a copy to zombie but realized some other readers might be interested so am reposting it here.)
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In the first three drafts of ANH, one of the repeated motifs is a scene where a heroic but aged character slams down his left arm on a hard tabletop, cracking it open, spilling wires everywhere and revealing it to be a cybernetic prosthesis. In the first draft the character is Kane Starkiller, in the second draft it's Han Solo's copilot Montross Holdaack, in the third draft it's Ben Kenobi himself. (In the first and second drafts, Kane and Montross both say that their "head and right arm" are the only truly human bits remaining. Ben Kenobi in the third draft presumably has somewhat more of his original body left.)
Here we have an outwardly-normal prosthetic arm, though inwardly mechanical, sported by a heroic character (usually a Jedi--but I suspect the sole reason for Montross' presence in the second draft script is this one scene). Also, it's the left arm.
Leigh Brackett's first draft of ESB doesn't have Luke being maimed. Nor does Lucas' second draft, although this is where the revelation of Father Vader first appears. It's only in Larry Kasdan's third draft that Luke first loses an arm.
According to The Making of ESB, the ESB third draft has three extra dialogue pages, Inserts A, B, and C, which describe Luke's traumatic injury and Vader's declaration of his parentage. These pages could of course be withheld from the script as generally circulated, in order to maintain secrecy. Insert B contains the "I am your father" line and is of little concern here.
In Insert A, we read that Vader 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 wipes the tears and blood from his eyes, but can still barely focus on his massive opponent."
Later, on the medical frigate at the end of the film, Insert C picks up: "Luke's lower left arm is exposed, revealing metal struts and electronic circuits similar to Threepio."
We can learn several things from this passage. First: Lucasfilm thought Luke losing a hand was a dramatic secret on par with Father Vader, and took steps to conceal it from the crew.
Second: In this early draft, Luke loses his left arm (just like the Jedi warriors of ANH). However, unlike the ANH descriptions of outwardly-normal, flesh-covered cybernetic arms, Luke receives a metallic, nakedly mechanical prosthesis. Basically, it's just like Anakin's prosthetic arm as seen in ROTS.
Also note the third-draft description of Luke immediately after his injury; he "can barely focus" on his opponent. Perhaps in this early draft, since he lost his left hand instead of his right, Luke actually retained his father's lightsaber?
Yet in the fourth draft, only Insert B (the revelation of Father Vader) is called for: no mentions of Inserts A and C appear. Thus, in this version Luke does not lose his arm--perhaps Lucas temporarily got cold feet?
Here Luke also doesn't lose his lightsaber: when he falls into the shaft, we see that "He has hooked his laser sword to his belt."
In the fifth draft/shooting script, there are no mentions of any special insert pages. Presumably this was done to avoid tipping off script thieves that any such extra pages existed!
Instead, the fifth draft description of the climax of the duel, and of Luke's recovery on the hospital ship, are seemingly complete, but in fact deceptive.
It is said that Vader brings his sword down on Luke's forearm, "cutting it" and causing Luke to drop his lightsaber, but he doesn't sever the arm outright. No mention is made of the "I am your father" bit, and no calls for extra script inserts are visible. Likewise, when Luke is being treated on the medical ship, special mention is made of the "nasty scar" on his forearm.
But these are obviously decoy passages, meant to deceive anyone not in on the actual facts of Luke's maiming and Vader's reveal. The truth would be evident only to those privileged enough to witness the filming on set.
One other note--there's a passage in a Making of ESB photo caption (p. 220) which suggests that the finished script actually called for Luke to lose his right arm at the elbow. Thus, in the final scene of the film, when Luke's new arm is being tested, we get to see robot components in his right forearm.
However, due to the limitations of special effects of the day, the shot of Luke actually losing his arm had to show it being severed at the wrist (so a false hand could shoot out from the cuff of his sleeve). Thus a discrepancy which persists in the final film.