When the hammer is cocked, the action slide lock spring allows the slide lock to sit up high enough to block the bolt slide/action bar assembly. See picture 1.
When the hammer has fallen, the curve of the slide lock spring pulls down the front tip of the slide lock to allow disengagement after firing so the mechanism can now be pumped. See picture 2.
Length, curvature, starting and ending rotation position all must be balanced to make correct operation.
I have to rework all kinds of obsolete mechanisms and happen to have some parts to fit these 520/620 etc. variants.