Congrats on the Ross acquisition, and I hope you're successful in acquiring all the bits to bring her back to military trim. Keep us posted on progress on that front.

As for the markings, you've got the Ross roundel, tje III to indicate rifle "Mark" and the 491/1915 OJ is your serial number w/ the 1915 indicating the year of manufacture. The over- and under-lined "70th" is a mystery to me, but my knowledge of these markings is very limited compared to others here. Perhaps one of them can shed more light.