Whistle for your frequency?

Thumbing through a 1962 FLYING magazine, I came across an advertisement
for the all-new Motorola M-400 NAV/COM. Among other quaint features,
such as the 100KHz spacing, later upgradeable to 50KHz, and the COM
transmit frequency that automatically switches to 122.1 any time you
select a NAV frequency "so you don't have to be continually switching
back and forth between your Com and Nav frequencies!" I am puzzled by
the all-new feature; Crystal Tuning so you can dial-in your frequency.
"No more whistling for a channel; just dial your frequency and you're
on!"

A cursory web search didn't reveal anything about this.
Who can enlighten me about this "whistling for a channel" business?

Thanks