As per the perfin, there are 8 positions a perfin may be found in. Upright, turned ledt or right, and upside down, and then also punctured through the stamp while the stamp was upside down, and thus mirrored.
Your image upside down, and then mirrored or flipped:

So could be a T or J lettering perhaps.
and a web site about Australia Perfins:
Perfin Stamps of Australia
http://www.perfins.com.au/and perhaps another site about teh SilverJubilee stamps' perfins:
King George V Silver Jubilee - Perfins of Australia
http://www.philatel2.com/jubilee/id213.htmthat could shed some light on something?