From 1978-79, these classic dispensers are known as "Softheads". There were only two other series made with these rubber type heads, Superheroes, which included Joker, Batman, Wonder Woman, Penguin and Batgirl. The other series was Disney. Disney Softheads never made it to the market and are quite collectible. One thing to note is that some of these dispensers heads are marked "Made in Hong Kong" and some just "Hong Kong". Sometimes different looks come with the different markings. I've pictured these differences below.

Are they not the greatest? Especially that pink stemmed guy!

Diabolic, Spook, Scarewolf, Vamp, Air Spirit Zombie
Different Head Markings? And colors?

Here's a look at the differences between heads marked "Hong Kong" (left) and "Made in Hong Kong" (right). Also note the "cartridge" that the rubber head attaches to. This cartridge fits onto the inner sleeve, similar to Croc (another favorite of mine).
*The Experimental Era*

These dispensers will most likely be found on Patent 3.8 (3,845,882), USA stems. This was PEZ USA's "Experimental Era". Many cool colored dispensers will be on the same stems.
Gosh Rob, those sure are great looking. Let's go see more Pez @ Spectres Pez Page