SPECIMEN on reproductions of art on stamps

I received an envelope from Australia today - I'd bought a first day cover featuring a cachet of Amelia Earhart - and on that envelope was a $2 stamp.

The text, below the picture of a woman sitting with her back against a tree, looking out at wilderness, is:

Fred McCubbin, On the Wallaby Track, 1896. Art Gallery NSW.

And on the right hand side of this stamp, in red, is the word SPECIMEN.

And I'm wondering, what is that word doing there? Does it have to be put on all reproductions of art so that somebody doesn't attempt to blow it up and make a forgery out of it? How could they do that to a teeny tiny stamp - blow that thing up to 3 feet by 5 feet and it would just be a blur!

So, why SPECIMEN?

And as you can see from the scan above, the two stamps on the envelope did not receive a franking stamp indicating they'd been used, but rather someone just drew a line across them with a blue marker.

What's up with that??

In doing a google search on this stamp, I came across this page:

http://www.stampcollectingblog.com/australian-fine-art-stamps.php

It shows the stamp without any word SPECIMEN on it.

So my search for enlightenment continues.