Question about some strange pieces in Stronghold package

[simmeone]

Hi,

yesterday I've got my new Stronghold for GF. Very cool gadget, but inside of the package is a strange piece, I do not know for what this is, take a look to the picture. Has anyone any clue? Thank you.

[Victorus]

Insets.
Look here https://yadi.sk/i/4aElPrRJs7WlLA
See section 5 Assembly order.
...Compensate internal plate thickness using
insets pos. 11 (Fig. 5.12).

[simmeone]

Aaah, thank you!