That is a sutherland MkI sight. It was introduced c1904 and the Cdn govt bought some for their Long Lees and they were also fitted to Ross MkII** and MkII5* rifles. The sight base on the MH, MM, ME, MLM, MLE and the above Rosses will take this sight. The sight base on the MkIII Ross will also fit this sight.
The Sutherland sight was also made for commercial sale.
IIRC the MH will shoot high with this sight which is probably why the extension is on the fore sight.
The first Nepalese MHs appeared on the surplus market in Canada in the early 60s which would give ample time for an enthusiast to fit this sight.