I just found out where I had 'swiped' that image of the pin. It was from a post to this 'site by member Deeangeo on July 27, 2015. Here's another from the same post:

The pin simply pushes into place and clicks shut. The notch in the sprung portion is pressed outward by spring tension to engage the thickness of the escutcheon plate. To remove, press the knurled part of the sprung lever with finger or thumb and pull out pin.

To 'take down'; remove magazine, turn lever in front of trigger guard, pull forend pin (as described above), lift barreled action up and out.