I think I have now worked this out.
The NLS historic 25" georeferenced map first looks on the server for
tiles from the County Series maps.
If that returns a 404 Not Found error (presumably because the sheet
wasn't available when the rest were scanned), it then looks on the
server for the same tile from the "Holes England" map to fill in the gap.
These appear to be from later OS revisions, but are available only for
the locations shown as blank patches at:
https://geo.nls.uk/mapdata3/os/25_inch/holes_england/#holes_england_new/ol3
Hopefully if/when they get enough of these scans, these later 25"
revisions will become available as a separate map on the web site.
cheers,
Martin.
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