Leonardo Notarbartolo's Inspiration

After hearing about George Leslie in Ron Faridmann's "Decoding Greatness", Wikipedia says this :

The most famous robbery that they pulled off was the Manhattan Savings Institution robbery of October 1878. It took Leslie three years to plan the robbery

Aha - if you read Greg Campbell's "Flawless" - about the Antwerp diamond heist, you learn they used similar methods of reverse engineering - in that case, two years of being a "customer" just to scope out the place and plan. And who's to say they didn't have an accomplice who had gotten on the payroll to help out?