Probably the reason Lucas had a skeletal Terminator style arm in AOTC was because in that brief shot he wanted to make it unmistakable to the audience (including those who may not have seen the OT) that it was a "robot arm."
If it had been a perfectly flesh looking deal, it would leave the impression that it might have been simply surgically re-attached or cloned tissue. It just leaves a more unimistakable impression than some servo sound effects to indicate cybernetics, which might be very audible over the loud music score.
Non-SOD of course...
It sounds likely that he "upgraded" the arm sometime in those 36 years.
Anakin's arm *Spoilers*
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