There is a significant numbers of ASA5510 chassis that have been reworked to look and act like ASA5520 or ASA5540 firewalls in the open marketplace and look exactly like the genuine article. Refer this bug toolkit for more details – text here:
Since the release of Cisco ASA software version 8.3(1) and the related recommendation to upgrade the RAM in all Cisco ASA-5520s and/or ASA-5540s, some Cisco ASA customers have contacted the TAC reporting devices with only one DIMM socket.
Working with these customers, Cisco conducted a detailed investigation and identified the affected devices as Cisco ASA-5510s that were modified post-manufacture to look like Cisco ASA-5520s or Cisco ASA-5540s. Cisco has always manufactured these products with either 4 or 2 DIMM sockets, so these modifications were completed without our consent and outside of our control. Cisco takes this situation very seriously and through our Brand Protection team is sharing these findings with the appropriate authorities.
Cisco’s commitment will always be to quality products and supporting our customers. That is why we also offer service and support contracts for products purchased from authorized Cisco partners.
If your device was purchased from an authorized Cisco partner, it will be supported.
In other cases we encourage you to contact the original point of purchase to determine next steps”
A Cisco authorised partner will be required to replace the unit. If you have purchased it from an unauthorised channel then you will need to buy a new unit.
Article on Cisco Support Forums ASA5520 Memory Upgrade where it seems the problem started in April 2010 and has continually gotten worse since then. Includes pictures and other interesting details.