What are Chinese banker's anti-fraud numerals?

I'm a 2nd year Chinese student and looking through the dictionary I found that 念 is listed as a "banker's anti-fraud numeral corresponding to 廿, 20." So what exactly is an anti-fraud numeral? Searching the web I didn't find any useful information about them, just lists. I'd also like to know a little bit more if possible, like how they came about and if they are still used. Thanks