Ted;
You say your understanding was this happened long ago. The Marlin 36 & possibly early 226 guns did not have the safety button. The only safety they had was the half cock. I truly don't want to sound too Calloused but anyone letting a "Loaded" gun down from a tree stand muzzle up likely deserved whatever they got.
I really don't know how this happened & seriously doubt the fact it had an external hammer had much to do with it. With the gun going down, anything snagging either the hammer or trigger should have been working in the opposite direction for firing. It, of course, could have been possible the gun snagged & he pulled it back upward a bit. If the hammer was then pulled back & released it should have caught in the half cock unless the trigger was snagged simultaneously. If just the trigger was caught he would have to have given it a pretty Stout Yank to have broken the half cock notch to allow the gun to fire.
