I'm not familiar with that stock release, never seen one before. I believe you are correct in that it is a tool, sort of like the third hand to remove a drum magazine.
I agree it should have been mentioned in the manual.
In Memory of Paul "Dietrich" Stines.
Dad: Say something nice to your cousin Shirley
Dietrich: For a fat girl you sure don't sweat much.
Cue sound of Head slap.
RIP Muggsy & TMan
"If you are a warrior legally authorized to carry a weapon and you step outside without that weapon, then you become a sheep, pretending that JOCKO will not come today."