On second look the T in the TZ mark on the front of the bolt handle is more like an I. So it's really IZ, or ZI. Still don't know what it might mean, or how common this is.

Here's something related to the center punching of the rear sight from Page 50 of the Ross Rifle Story,
"About this time, Ross suggested discarding the windgauge of his screw elevating rear sight. The idea was considered and dropped as was his request to have the sight replaced by the new Britishicon War Office pattern made up for the 100,000 Mark III B Rosses Britain had ordered September 15, 1914."
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