Nut Ive not noticed before

While trying to make my thower as screen accurate as possibal ive seen this nut on this picture

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Ive looked about but couldnt see any thread’s about this

What do you guys know about it?

It’s there on that gun…

There are other misc screws and sorts on different guns…

Oh and BTW it’s not a nut but allen head (socket) screw…

I put it on my new gun. I’m pretty sure it is mentioned in Stefan’s plans.

I forget what it did.. but I think it had a purpose on my gun:

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It would seem to me to be a perfect place to attach a mounting block on the inside for the bottom plate’s middle screw on that side.