SWAPEROO is a now-defunct academic exercise to write a highly flexible wallet. It has a lot of high level documentation, but the code never got out of technology demonstrator phase.
Their terminology uses Instrument as we would use Contract or Wallet. To them, a Wallet is what we would call WebFunds, an application you can plug multiple "instruments" into, each handling different payment classes or technologies.
I'm not sure on the precise difference; in this, it seems that the Instrument is similar to the use by Intertrader.
Latest status is that IBUC were digging out the code. Unknown licence. If they ever get it working, it will be worth looking at to see how they've done things.
It would be nice to write papers like their's. Check out the project doco: Digital Wallets Project Home Page and Javadoc and Original Paper for 3rd USENIX Workshop on Electronic Commerce, November 1998.