Type 99 LMG unknown loose part

[nate129]

My type 99 LMG had an anomaly when last on the range, after a few rounds on the second magazine fired the ejected brass was mangled near the mouth of the cartridge. This gun has always been pretty gentle on ejected 7.7x58 in the past. I stopped shooting and took it home, the pictured part fell out of the receiver when I removed the gun from the case. The part slides in and out freely. The geometry of the part only allows it to be slid in from the left side of the receiver. Appears to be an easy fix as the previous receiver marks seems to indicate it is simply staked in place. I have a few books on this weapon including a copy of the WW2 manual in Kanji. I can't find the pictured part in any of these documents. The part is stamped on the lower side which matches the receiver's serial number. Does anyone know the name for this part and what its function is?