I believe you have it reversed. The first column is the locations on the map and the second is how they're listed by the game. So if you're missing piece #7, it'should be in location #8 on the map.

@Reignfire Your list is inaccurate. I used the command, and it says I'm missing sword piece #7, which (according to your list) is piece #2; which you literally cannot miss, as it's given to you by the robot.

Why the video has a missing piece? Number 11, underwater dungeon, where the fck is that

the video really helped, thanks a lot. I would heve never found the one in the dungeon.

Dear MackNJeeves and Reignfire: Thank you both so much for your great work and tipps. This helped me to find the one piece of sword I missed without having to mindlessly search for hours and revisit all the found ones again. Great job you two!

Sorry guys! Link to the video must have been removed or been broken. I re-added it!

If you need to figure out which pieces you are missing, this will help:
Press "~" to access the console and type "list swordpieces" which lists the pieces you do and do not have. The map numbers 1-12 do not match the list numbers 0-11 so I made a chart.
01 = 09
02 = 10
03 = 11
04 = 08
05 = 00
06 = 01
07 = 02
08 = 07
09 = 04
10 = 06
11 = 05
12 = 03

I am struggling to figure out how to access the area for sword piece 12. It is the only sword piece that doesn't show how to access the area on the map in the video. Anyone know how to get there? I have opened up all the water passages, but the area indicated on the map is just a sheer cliff.

im at the finale before final boss and i want fully upgraded sword so i want to go down but i cant get down from the sky base anyone know how?

Perhaps it's fixed now, I see the video link. Here it is just in case:
https://www.youtube.com/watch?v=Vx5oKTS54CY