This is the top plate of the extended mag for the Glock AEP, such as tokyo maruy G18C or Cyma_cm030.
It is a very small piece, consider printing at low speed (I printed it at 20).
To make it work:
Once printed, smoth possible imperfections and smoth the tracks where the spring and the latch go. With a large drill bit, make a chamfer for better sitting the screw. Finally, don't tighten the two screws to hard, tightening to hard prevents the BBs arrange properly and therefore no BBs come out..
Also, make sure the plastic extended mag doesn't open when is full loaded of BBs. I overcome this problem by melting the plastic with a soldering iron. It can be pretty ugly, but it works great.