Its the forging mark. Keyhole means CERRO FORGE.
Cerro forge marks have been seen on COLT, dpms, bushmaster, etc etc etc....
The forger supplies rough forgings to companies that mill the part to spec.
A (splintered) = Anchor Harvey Aluminum
AF = Alcoa Forge
C AF = Colt Alco Forge
C MB = Colt / Mueller Brass
Cardinal (stylized) = Cardinal Forge
CH = Colt Harvey Aluminum
CK = Colt / Kaiser Aluminum
Circle/Crooshairs w/"AR" = ArmaLite
CM = Colt / Martin Marietta
D (stylized) = Diemaco
DK = Diemaco / Kaiser Aluminum
E = Emco
EK = EMCO/Kaiser
E MB = EMCO/Mueller Brass
F keyhole = FNMI / Cerro Forge
FA = FNMI / Anchor Harvey
FK = FNMI / Kaiser Aluminum
FM = FN/Martin Marietta
FMB = FNMI / Mueller Brass
Keyhole = Cerro Forge
L = Lewis Machine & Tool
LK = LAR / Kaiser Aluminum
LM = LAR / Martin Marietta
M (under diamond) = Mueller Industries
PA = Capco / Anchor Harvey
PM = Capco / Martin Marietta