I am sure the experts on Australia will be of more help here. But the watermark would be hard for me to ignore.
You could remove all doubt by buying another #300, as it is only a couple of bucks I believe. Just looked on eBay and there is a nice used one for 1.79.
Thanks Rob. Looks like the Perfs can be a bit more fluid (15 = 14.75 ?) so I think the ultimate test is really no watermark for #300. Will pull out all the little buggers out of my shoebox and have another go at them. Fortunately found another envelope of oldies last night that had a dozen more.
Thanks Rob. I have a 13.25x... 15x... and Unwmk version of each of the 1930s/40s definitives that I found so I'm stopping there.
I find Wmk detection to be particularly unenjoyable. Unfortunately finished going through all the piles last night and have probably 8-10 old 'Roos and maybe 40 George V's.
I put them aside as I can't bear the thought of the same agony checking-agony. Probably disturbing to "real" Australia collectors (and I understand there could be some decent CVs in there) but I think I'm just going to keep one of each denomination and the rest will go on the block...