on-middan
This might be just a supplemental entry adding to an entry in the Main Volume.
In the OE text, the length is:indicated by acute accentsindicated by macronsnot indicated.
on-middan
Add:where the governed noun comes between on and middan Þá wæs þǽr án mycel burh on heora wege middan, Hml. S. 25, 440. Hé wearð forbærned on þám bæþe middan, Hml. A. 60, 209.
This is a supplementary entry with editorial changes to an entry in the main volume of the dictionary. Look under the 'Possibly connected entries' below, or for the same headword in the list to the left.

A Combined List of Abbreviations.

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A Combined List of Abbreviations.