A truly decent audio oscillator was only available after 1939, when Hewlett & Packard issued their first instrument.

In a 1985 letter, Dave [Packard] described Hewlett's audio oscillator as "the foundation on which Hewlett-Packard Company was able to grow into the largest manufacturer of electronic instruments in the world, the keystone that allowed four and one-half decades of major contributions to electronic measurement technology and equipment."

Maybe but it'd have been significantly more complex, here they could take a MHz piezo-electric crystal and tack on a few electronic multipliers and dividers (/1000 *11 /25) to get the desired frequency.

Quartz oscillators are usually made in at leas 10th of kHz range, for whatever reason. Second most stable oscillators are LC and would require large coils for lower frequencies.

It would still have been divided down. There aren't many "Hz" oscillators. They're all usually in KHz/MHz range.