It seems to draw some inspiration from Soviet anti-tank rifles, PTRD and PTRS, if nothing else...
PTRD-41:
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That drawing of the PTRD is a bit wonky.
The cheekpiece should be on the left/port side.
The plate that forces the bolt open upon recoil should be on the right side
The little hook at the rear of the receiver is the end of the firing pin. It is correct, but there is no bolt handle shown.
In your last photo, the gun on the right is a PTRD. The action is open and the bolt handle has ridden up the plate to force the action open.