on-mang
This might be just a supplemental entry adding to an entry in the Main Volume.
Related §§ in Wright's OE Grammar:
In the OE text, the length is:indicated by acute accentsindicated by macronsnot indicated.
on-mang
AddHé ásende his lác onmang óþra monna lácum, Gr. D. 230, 20. Syðdan hé hæfde gewunad onmang mannum þreó and xxx wintra, Wlfst. 292, 11. Rihtgeleáfa ásprang onmang Godesgelaðunge, 294, 3. Onǽlde his blácern onmang þǽre þearlan hǽtan þæs lustes, Hml. A. 196, 44. Onmang þisse sprǽce, Angl. viii. 308, 34.
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A Combined List of Abbreviations.

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A Combined List of Abbreviations.