The source from the article doesn't sound like it's a measure against tampering, but instead a way to stop the patient receiving the wrong dose by accident. Prior to the early 1900s I'm pretty sure you could buy cocaine and opium over the counter in most of the English-speaking world, so there wouldn't have been any reason to alter your prescription.

Edit: the source given by the article was from 1919, so it would have been within a couple of decades of the earlier laissez-faire drug policy.