The number I gave is only a kind of rumor. I never checked its veracity. Maybe 1.5 million prints is too much for a set of printing plate...But there is another possibility: that there is not one but three sets of printing plates on the revolving drum of the press. Remember that the tiny numbers are POSITION numbers, not really a PLATE numbers, so the position number is only a reference for the pressman to spot quickly to a position any problem found on a note during QC. These is no direct way to know if there is two or three identical printing plates bearing the very same position number.
And I found in many instances the evidence that there is three sets of printing plates since I found tiny errors (blank spot, default) on not consecutively numbered notes, but most of the time, on each three notes.
For example: a white dot on the reverse can be found on the note ABC****001, then ****004, then ****007, and so on, but the notes ****002, ***003, ****005, ****006, etc doesn't have the white spot. The pattern is not perfect (since we cannot know exactly how many defective sheets are taken out by the quality control before the serials are printed on the remaining sheets).
If I'm right, we can conclude that a set of 40 plates can print around 500 000 to 600 000 sheets, since a given set prints only one sheet each three. This number is more in the order of the number you gave (nearly 200 000 prints 40 years ago), notwithstanding the improvement in the printing technology.