The CANCEL mark was a dash, usually with a small dot in the middle. It was used for cancelling other markings.
The SALE MARK, signifying that the rifle was legally Sold Out of Service while they still were being issued, was two BROAD ARROWS, point-to-point.
This looks like a Sale Mark which has been Cancelled: something I have never seen before.
POSSIBLY the rifle was up for legal sale, then was recalled for Service, likely when the Second War was, at last, recognised to be inevitable. Britain was demilitarising, selling, giving away and destroying First World War equipment up until late 1937, as far as I can find out, and possibly even later. Nobody wanted to upset that nice Mister Hitler, who had been re-arming like crazy since about January 31, 1933; he became Reichskanzler (Prime Minister) on January 30.
This would fit with the other markings on the rifle, I think. Besides, it is the only solution I can think of. As a single marking, this one isn't in any of my books, not even TBSA - 1909, which is about as complete as they come.
The = sign right beside the III would indicate that the rifle was converted to III* configuration at some time. This was generally done to Mark III rifles when they came in for servicing and rebuilds from about mid-1916 onward. AFTER the end of the Great War, the standard reverted to the III, which meant that rifles with the cutoff slot got their cutoffs back, along with volley sights nd the appropriate woodwork. So it was a Mark III rifle, converted to Mark III* (removal of the cutoff and volley sights), restored to Mark III* status...... at which time the = was stamped on, CANCELLING THE *. The = was only used for cacelling the *; anything else needing cancelling got the Cancel Mark (_._) or a series of //// to blot out old numbers. You see this often on the backs of rear-sight leaves, nose-caps etc., when they were refitted to a different rifle during rework.
Hope this helps.
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