I learn more from answering questions, sometimes.
the size is more accurately 12.4 and has small holes from a second machine 1903-
That may be so, most monographs go to the quarter perforation Guage
I ahve not read much on particular machines.
Your stamp appears to be Line Perforate 12.5 (The corner teeth do not support Comb Perforate)
Western Australian Perforations.
(The WA Study Group) Page 405