The source code is indeed missing as etherscan cannot link it easily.
If you switch to OPCODE View, you're going to find the following:
Judging by the time the contract was deployed and the code it's using ( CALLCODE ) we can determine that it's actually delegating it's storage and execution to another contract.
Now you have the actual ABI and source code that you can use to create calls against it.
These days instead of CALLCODE, we use DELEGATECALL for such cases.
Hope that helps.