An ornamental fret or decorative pattern, possibly one inspired by or suitable for calfskin leather binding.
Possibly from 'calf' combined with 'fret' (a decorative design), though this may be a specialized or archaic term from bookbinding or decoration terminology.
This could be a bookbinder's technical term—decorative patterns had specific names, and 'calfret' might describe ornamental designs commonly tooled onto calfskin book covers!
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