Misprint 50 Dollar?

It's not a misprint. The ones without the name are simply from an older series.

When the bank first issued them, they apparently believed that the signatures of the estimable personages they chose to put on the notes would be sufficiently legible for people to read. Unfortunately, they're not. So in the late 1990's, they decided to add in small print the actual names of the persons depicted.

Unless they're in absolutely pristine condition, neither note is likely to be worth keeping.

You can confirm the banknote' age by looking at the serial number, the first two numbers (after the two letters) are the last two digits of the date of issue. The note without the signatures is the older note.

As you can see by my comment in this thread, this sometimes even trips the locals up, making them think they've got a counterfeit note or something when all they have is an older note.

G'day,
quote: "I am about to go to Australia on holiday ..." - have a great trip.
quote: "... just ignore it and spend it on beer?" - be careful: the beer here is COLD.
Peter in Darwin
in the tropical north of Oz