Yep, it's a PA15 or PA20. I have a PA20 and it sounds just like that. The PA15 and PA20 have wail, yelp, and alert tones and were manufactured from about 1962 to 1966.
The movie and TV studios in Hollywood recorded the sound of a PA20 during the late 1960s, and it's been dubbed into hundreds of shows and movies throughout the 1970s.
Finding a PA15 or PA20 in working condition could be kind of tough - they weren't very reliable and there were several minor revisions to the circuitry over the course of production. My PA20 looks like it's been repaired a few times.
The 'bomb drop' effect may have something to do with the layout of the siren oscillator circuit. The 1970s PA15A 1E and PA20A 2E will also make that sound (especially when switching back to wail from yelp), but they are almost an octave higher in pitch. The PA15/PA20 and the PA15A1E/PA20A2E use similar components, but not all values are the same. The high pitch is what really sets the 1970s Federal electronic sirens apart from the 1960s versions.
Also, the PA70, PA100, PA150, PA170, PA200, and PA1000 have the same siren oscillator circuit as the PA15A 1E and PA20A 2E and should be capable of the bomb drop effect.