This was a little pricey, but I needed a copy for my collection. Sideways watermarks are rare, but not really for this one. Identified as Scott 51b, ACSC 33aa, Mint Never Hinged.
Through Scan showing full sideways watermark, plus a partial one:
I wonder about the vertical perforation spacing at the top of this stamp. It appears to be different than all of the others. Any comments from our Aussie friends?