Question about grenade shell

So I have a Madbull XM204HP (204rd) airsoft grenade shell and I was wondering where i have to put the ball bearings. Yes, I know that the top is missing an o-ring, i already bought it along with the usual 4 ball bearings that Madbull shells need. If you could describe with detail or copy the picture and draw an arrow to where I need to put the ball bearings with Paint, then I would appreciate it. Im new to grenade launchers and the ball bearings were lost by the original owner.

Anyone have any idea????

Bring it to pb tomorrow ill show you.