hap, both Mondial solve a problem in the same time, I measured both near an AM-Radio, and the frequency is indeed near 1MHz.

As for the active buzzer, look here:
https://www.addicore.com/Active-Buzzer-5V-p/146.htm

Edit: The beep could also be generated with the 74LS14 and 74LS74 next to the silver inductor (or XTAL?) top left.