Walk ENTITY-MIB::entPhysicalSerialNum (1.3.6.1.2.1.47.1.1.1.1.11),
there are several entries: chassis, radio, ...
Part of entPhysicalTable for AP1242AG
ENTITY-MIB::entPhysicalDescr.1 = "Cisco Aironet 1240 Series (IEEE 802.11a/g) Access Point"
ENTITY-MIB::entPhysicalDescr.2 = "802.11G Radio"
ENTITY-MIB::entPhysicalDescr.3 = "802.11A Radio"
ENTITY-MIB::entPhysicalDescr.4 = "PowerPCElvis Ethernet"
ENTITY-MIB::entPhysicalSerialNum.1 = "FCZXXXXXXXX"
ENTITY-MIB::entPhysicalSerialNum.2 = "GAMXXXXXXXX"
ENTITY-MIB::entPhysicalSerialNum.3 = "ALPXXXXXXXX"
ENTITY-MIB::entPhysicalSerialNum.4 = ""
ENTITY-MIB::entPhysicalModelName.1 = "AIR-AP1242AG-E-K9 "
ENTITY-MIB::entPhysicalModelName.2 = "Radio AIR-AP1242GR:5M80.13"
ENTITY-MIB::entPhysicalModelName.3 = "Radio AIR-AP1242A:5M80.13"
ENTITY-MIB::entPhysicalModelName.4 = ""
OIDs numerically (entPhysicalSerialNum only)
1.3.6.1.2.1.47.1.1.1.1.11.1 = "FCZXXXXXXXX"
1.3.6.1.2.1.47.1.1.1.1.11.2 = "GAMXXXXXXXX"
1.3.6.1.2.1.47.1.1.1.1.11.3 = "ALPXXXXXXXX"
1.3.6.1.2.1.47.1.1.1.1.11.4 = ""