Occasionally for some Cisco devices Spectrum displays the Serial Number field as empty even though the serial number is available in the mib.
Spectrum by default gets Serial information from DeviceSerialAttr attribute 0x3d0063
It points to attribute 0x210b5d ChassisId:
ChassisId 0x210b5d Text String 126.96.36.199.188.8.131.52.6.3.0
184.108.40.206.220.127.116.11.6.3.0 is included in OLD-CISCO-CHASSIS-MIB
For some Cisco devices a serial number could be populated in another way.
Set attribute DeviceSerialAttr 0x3d0063 to point to attribute 0x12a6e NRM_ChassisSerialNumber.
Right click on the device and choose reconfigure model.