The PA15 (Director) and PA20 (Interceptor) both have wail, yelp, and alert tones. These sirens were in production from about 1962 to 1966. Both sirens have low-pitched wail and yelp tones. There were nine variations of each model. I have manuals for PA20 series E1, E1A, E1B, E1C, E1C-C, E1D, E1D-J, E1D-S, and E1E. The siren oscillator circuit remained roughly the same despite the revisions, but I think it's safe to say that these sirens were unstable if the circuitry needed so many revisions. If you have one in working order, treasure it. PA15s and PA20s are not that easy to find, and working units are even more rare.
The PA15A has wail and yelp tones only, while the PA20A has wail, yelp and hi-lo tones. The PA15A and PA20A were introduced in 1967.
Late 1960s PA15As and PA20As (A, B, C, and D series) have low-pitched wail and yelp tones. There seem to be more D series versions around than earlier versions, so I assume that production continued into the very early 1970s. These sirens are not that common either, and after having gone through some revisions they must have had issues as well.
1970s PA15As and PA20As (E series) have high-pitched wail and yelp tones. These are the most common versions of the Director and Interceptor. Series 1E Directors and series 2E Interceptors are very easy to find, but bear in mind they are 1970s models and would be best suited for post-1972 emergency vehicle restorations.