Here the turning motion of the knob rotates a diagonal rod inside the "blind cap" which in turn forces down the cam at the end of barrel and this motion pushes the squeeze bar inside the barrel.
At the end of the turn the cam structure release the whole mechanism back in it's initial place. All this is accompanied by a pretty loud "click" . Much more like a "button filler"
I've taken a few pictures of the barrel end with the "blind cap" off. Hope it helps to better visualize the filling mechanism:
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