You would be surprised how easy it is for the government to track cash. If the bills had entered circulation, they would have been detected. They would have long since worn out and been removed from circulation. The serial numbers would have been recorded and detected long ago.
Now there's a magnetic ID strip in the bills, there wasn't in 1971. And it's easy to forget how automated things are in general compared to then. Yes, it would have been possible for people working manually to do it, it just wasn't done because the activity that would flag bank employees to do the check -- large deposit -- didn't take place within a reasonable timeframe after the robbery. This new suspect did open a new acc't with a large chunk of the money, but he did it in Canada.