What am i missing??? Detent on PMI-1

[Junglepeanut]

I am not a brass guy or a stock class guy so i am sure i am ignorant and missing something obvious. I just stumbled across this and have never seen a setup like it. What is the purpose of this detent behind the breach? At first i thought it was there to use as a stick feed from the rear but nothing triggers the detent to allow a paintball to load with each shot unless there are pieces missing. Is this a common mod or some home brew thing..... What am i missing?

[Stilgar]

That's an old speed feed. You use your finger to move the detent out of the way when filling the top tube.

[Junglepeanut]

That was my other guess, kinda double up your ammo on board...... thank you for the info