I'm reminded of a story in the Book Of Heroic Failures which went something like "Mr Smith, who attempted to pull off the worlds largest single cheque forgery, altered his pay cheque from 14 pounds 8 shillings to 148 million pounds. The fraud was successful right up until the moment he tried to cash it."

The best security for thousands of years has been simple - be a suspicious bastard.

Other than the one to two transformation, are there others? Was the formal system a response to these problems or how it has always been done?

The more formal number ideographs are the original ones, the simpler ones were simplified so numbers aren't as awkward to use.

You can also easily change a one into a ten, for example. Note also that it's not exactly a positional number system like Arabic. E.g. the number 123 would be written 1 hundred 2 ten 3. IIRC you can also easily change a ten into a hundred or thousand, so for larger values with a few zeroes in between you could shift a meaningful place into the zeroes to the left sometimes.

Well, just look at the symbols in his comment. 2 into 3, 4 or 5 is pretty easy looking, as is 1 into 10, 7 or 8. I'm sure you could probably spot more.

Very interesting, thanks - does it apply to Japanese, too?

Yes, with very similar characters. Although in practice only those actually needed to resist forgery are commonly used: 1: 壱 2: 弐 3: 参 10: 拾

See https://en.wikipedia.org/wiki/Japanese_numerals#Formal_numbe...