Comment by jerf

You shouldn't really need to prove where you got them from; what the system cares about is that you're not laundering it, and it's obvious that nobody launders money on a multi-decade timescale. All you should need to satisfy them is just to show that you've been in possession of it for so long the bank can't even show where it came from anymore. That is itself proof that it's not laundering. You may have good success just pushing back a bit with this point.