I've never heard of a series 2F, unless the 2F is the 24 volt version of the 2E.
If you want a PA20A there are basically two versions (identified according the sound of the wail and yelp tones).
Early version (series 2B, 2C, and 2D): low-pitched wail and yelp tones - introduced in the late 1960s.
Later version (series 2E): high-pitched wail and yelp tones like the electronic sirens we hear nowadays - introduced in the early 1970s.
Series 2E is by far the most common version and is easy to find. If you like the high-pitched tones, you should be able to find a series 2E for sale real cheap.
The late 1960s versions with the low tones are rarer and worth more money.