looking to get the Stanley Gibbons Australian digital one as soon as I can afford it.
Good luck with that, not my choice.
Pick up a kicked and abused Gibbons on eBay
for pennies, if you wish.
I employ Scott Catalogue, for my worldwide.
Your stamp is Sc#221 1905 Type A50.
It was carried aboard a cover that was Registered, identified by the red wax crayon and the large "R" cancellation.
Yes Wmk V over Crown
Type 46 in the image above.
I'll consult my well thumbed Gibbons for anything further.
Gibbons#359 1901 dull blue
Wmk : Gibbons do not specify in my specialist catalogue on rotation, they deem it unnecessary as the sheets could be fed in either direction.
Specialists here in this forum may offer a differing opinion.
I would mount the stamp with your observation, or lightly pencil "inv" on the back.
Readers will probably be up in arms, but I feel it better to mark the stamp than lose it later.
Mounting and writing then best husbandry. Do not do an a mint stamp.
Is it a rare thing to be upside down back in those days or did the printers just pop the sheets in whichever way?
Wmk identification and rotation, is a labour of love, and will challenge you as a stamp collector.
It can result in a stamp worth thousands of dollars, but its like panning for gold, you have do do 1000's of hours of donkey work, and even then it may be fruitless. It's up to you.
Main Wmk ID is on the Kangaroos and KGV sidefaces, then it is a "must"