I'm enabling a cluster of ASA 5520 and after insertion of the appliance 3DES key was with ASA 5510 features For example, the interfaces are FastEthernet and GigabitEthernet rather than as native appliance. See the "sh ver":
ciscoasa# sh ver
Cisco Adaptive Security Appliance Software Version 9.0(3) Device Manager Version 7.3(1)101
Compiled on Fri 19-Jul-13 16:37 by builders System image file is "disk0:/asa903-k8.bin" Config file at boot was "startup-config"
You will have to work with Cisco Licensing on this to resolve. I have had to work with them multiple times to get the right key so don't be shocked if you have to try several keys they send you. Perhaps someone has run into this exact issue and can give feedback but stay on Licensing to get this straightened out.
When it comes to licensing there isn't much we can do outside of speaking with Cisco. Again, stay on top of them. I tried to get a Security Plus license because they shipped an ASA with Base License, the first key gave me Security Plus but features were still missing, 2-3 keys later and I finally got all functionality back.