VKB-SIM

What are the extra parts for?(I know about the springs for different tension, but what about the larger metal parts?

You might be talking about the "cams" which allow you to change the feeling of the center - either soft, with no detent, or less soft, with a tangible center detent. It's really up to your preference which one you like best. Me personally, I always go for the soft center type cam as that feels more "realistic" to me while flying.

When assembling, disassembling parts of the gimbal, including taking springs off and exchanging cams, be careful, work slowly and deliberately, and use the right tools!

Check this video for reference:

https://www.youtube.com/watch?v=_x6RdEJJ_PM&feature=youtu.be