Mysterious thing for TM HK416D

[Sam Pascoe]

Just installed a new barrel and rubber in my 416, tested it and it all works fine, but after returning to my desk i found plastic clip. 
I would like to know where it belongs, if it is important and what does it do? 

many thanks 

 

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[Esoterick]

Look like the spacers that sit between the receiver and the gearbox shell, should be one on either side where the receiver has a bulge running along it.

 

[Sam Pascoe]

you are correct, thank you very much