I'd be more suspicious of the effects of psychoacoustic compression than of frequency response.
Even 60s half-decent recordings almost certainly had above 10kHz, likely 16kHz.
Grundig's 3-3/4ips recorders from the 50s claimed ~10kHz.

The very old 78rpm discs were typically significantly poorer, but by the 50s vinyl was taking over.


Jazz relative beginner, starting at a much older age than was helpful.
AVL:MXE Linux; Windows 11
BIAB2025 Audiophile, a bunch of other software.
Kawai MP6, Ui24R, Focusrite Saffire Pro40 and Scarletts
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