Well i would say you have a lemon M-box most likely from factory unlucky bad molding and cracked/fractured apart over time.

Replace ASAP to avoid further damage.

Edit:

I highly doubt it came from the factory like that. Most of these shells are cast and that looks like a perfect line across so I'm sure it was cut. The factory will surely catch something that big missing even if they have poor QC.

That looks like a dboys m-box. Someone cut that for some purpose and if it just fell out as you took the gun apart then it was intentional. Go bitch to the seller.