cstm is installed with the OnlineDiagnostics / Support Tools Bundle on HP-UX 9000 systems:
# echo "sc product system;info;wait;il" | cstm | grep -i "system serial"
# echo "sel path system\ninfolog\nexit"|cstm|grep "System Serial"
# SERIAL_NUMBER=$(echo "sc product system;info;wait;il" | cstm | grep -i "system serial" | awk -F: '{print $2}') # echo $SERIAL_NUMBER
# getconf MACHINE_SERIAL
HP-UX 11i v2 on Itanium-based systems, use the machinfo command:
# machinfo | grep "machine serial"