Which OID TO SNMP poll for the chassis number of an ASA 5585?

Trying to create a report that lists chassis Serial number for all my Cisco devices and have been using CiscoChassisSerialNumber and chassisSerialNumberString and chassisId to poll for chassis Serial number over a range of different Cisco devices successfully but when I try these on an ASA 5585 they're not supported. I did a MIB walk on the box both standard and private MIB walk and I can't see anything to poll for Chassis serial number. I can poll entPhysicalSerialNum but this does not give me back just the chassis serial number.