I find it odd that the 20 dollar bill they are using on that site is an "Old" 20...as the trick can be done with any 20 printed in such fasion,
which began what, 1930s? Maybe 50s...can't remember.
Nah, all bills were changed during the Nixon administration because he freed it from the silver standard. The older bills had a guarantee on it that
you could exchange it for silver. The $20.00 U.S. bill is the most counterfeited note in the world. That is why it has to be changed routinely.