In the first pic shown, notice the large pin protruding from the top and front of the trigger group housing.
This is an interlock which prevents the front takedown pin from being removed and the trigger group
housing being dropped from the rifle, when the bolt is locked into battery.  This prevents the accidental
discharge that is inevitable if the takedown pin were to be removed  and the trigger group housing
dropped while the rifle is loaded and locked into battery.

Now, here is the same pin with the stainless steel capture pin removed.  Not shown,
the spring for the housing lock pin, the housing lock spring is still in the spring well.

Look carefully at this photo.  Inside the ejection port, near the breach face, at the bottom,
there is a shiny spot.  This is the top of the housing lock pin.  When the bolt is thrown into
battery, the bottom locking lug rotates over this spring loaded pin, depressing it.

Now, please notice these pins.  All 4 takedown pins are identical.  There are springs
captured on both sides, front and back, of the takedown pin holes in the rifle.  These
springs snug into the depressions on the takedown pins, holding them in place.  But,
the forward trigger group housing takedown pin retention setup has an extra feature.
This spring, the housing lock spring, loads the captured housing  lock pin.  When the
bolt locks into battery, this interlock pin is depressed by the bottom locking lug.
This interlock blocks the forward trigger group housing takedown pin from being
removed, even intentionally.   A very, very good safety feature.