Key, 4x12 mm 

In your instruction manual, this part appears on the parts list as:

Item No	Models
124	Grizzly G0463 Grizzly Small Mill Harbor Freight 93885 Central Machinery Small Mill Various X3-US SIEG Small Mill Woodstock International M1110 Shop Fox Small Mill
167	LittleMachineShop.com 3536 HiTorque Bench Lathe - 8.5x16 LittleMachineShop.com 3540 HiTorque Bench Lathe - 8.5x20 LittleMachineShop.com 3595 HiTorque Bench Lathe - 8.5x20 Various SC4-US SIEG Bench Lathe Travers Tools OT218410 OTMT Bench Lathe - 8.5x16 LittleMachineShop.com 7550 HiTorque Bench Lathe - 8.5x20 LittleMachineShop.com 7500 HiTorque Bench Lathe - 8.5x20
2-27	LittleMachineShop.com 7600 HiTorque Bench Lathe - 9x20
40	Harbor Freight 47158 Central Machinery Micro Mill Clarke CMD10 Metalworker Micro Mill Various X1-US SIEG Micro Mill
6-58	LittleMachineShop.com 7600 HiTorque Bench Lathe - 9x20 LittleMachineShop.com 7650 HiTorque Bench Lathe - 9x20
79	BusyBee CX611 Craftex Small Mill Grizzly G0619 Grizzly Small Mill Various SX3 SIEG Small Mill Travers Tools OT2225SX3 OTMT Small Mill

Click your model to see the complete parts list.