Richard,
1) Make sure your pistol is unloaded. Remove the magazine.
2) Safety off.
3) Holding the pistol in your right hand, push the barrel against a hard surface such as a table top. When the barrel/upper assembly has moved to the rear about 3/8", rotate the take down lever to its bottom position. Remove the now loose side plate.
4) Pull the upper assembly forward and remove it completely from the pistol.
5) Unscrew the hold down screw from the left grip panel and carefully raise the lower end of the left grip panel by placing your finger into the magazine well and pushing upward on the grip panel. Carefully remove the grip panel. This procedure is to keep from chipping the grip panel at the safety lever area.
6) Observe the small flat concave spring that fits between the top of the knurled magazine release button and a small cut in the bottom of the receiver side panel. With your thumb slide the top of this flat spring to the front of the receiver. It will come free of the pistol.
7) Using a small soft piece of wood or plastic, if necessary, push the magazine release button through the pistol frame from left to right. It will come free of the pistol.
All of this is to let you see the nature and operation of the "pin that runs through the frame".
Reassembly is in reverse order with the caveat that the small flat magazine release spring must be installed "concave," as it faces you, so that it will properly operate to secure the magazine release button in place and provide proper tension in order for the button to operate. Also, be aware when re-installing the complete upper cannon assembly into the lower assembly that the "S" shaped stirrup found dangling at the rear bottom of the cannon assembly must be guided into proper contact in front of and under the stirrup receiver that connects to the main spring assembly.
I apologize if I've merely stated the known and obvious. I'm open to and welcome "Grading" by the real experts.
As always, best regards
John