Top is definitely one piece I think - my own theory is that it was cast when the Obi stunts were being cast (it would certainly be more efficient that milling those shapes)
Here's the best shot down the nose for ya:
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I think it may be a remnant of the original ANH set up - which was slid to the back to act as a counterweight for ESB (the brought back up when it was no longer for dueling) - I don't have any actual evidence for that though. I also see a tube in the middle of the hole - which would make it similar to the touring saber setups -
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I definitely think you're right about the milled step inside the shroud as well. That's what I did on mine! I should have made it up higher in collar as you said though - I'll probably be updating mine since it would make it more secure on the tube body - something like the 2nd pic
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