The AS heads were the first to start with and the last to be finished, having had a total remake. What they have that other heads don't, is separate ears. These were made to deal with long and pointy ears with or without jewellery. The heads come in a series of shapes from stretched to squashed for adults, and squashed to even more squashed for children. The older heads have single and double versions, and are not numbered sequentially because the first meshes got their numbers in the order that they were made.
The following picture shows all AS models, connecting with arrows the child heads that are simply a shrunk version of a adult head, and giving the same background colour to models with sufficiently similar UVmaps to share a texture.
The adult model mesh codes, single head first:
CAS3/CAS8 - short, round, wider, ridiculously big eyes (Urawa, most young female characters)
CAS5/CAS0 - triangular, big eyes (Zoisite, the Ayakashi sisters)
CAS2/CAS7 - oval (Mamoru, Motoki, young male characters)
CAS6/CAS1 - as before but longer (Nephrite, generic male characters)
CAS4 - first "freak" head: like CAS1 but wider chin (Yuichiro, so he can show off his stubble)
CAS9 - second "freak" head: CAS8 with puffy cheeks (Umino)
The normal heads come in single and double version, the "freak" heads are double.
Added later, both double:
CASA - very long face, small eyes, for mostly adult male characters like Sakkyo and Sensui
CASB - made off CASA but shorter and wider, for Bui
The child model mesh codes:
CAS3/CAS8 - short, very round, widest, ridiculously big eyes, like Chibi-Usa
CAS0/CAS5 - shrunk version of adult CAS3/CAS8, for Genkai
CAS7/CAS2 - shrunk adult CAS5/CAS0 with CAS3/CAS8 eyes/chin; generic child face (Shingo)
CAS6/CAS1 - shrunk adult CAS5/CAS0, for generic older child face
CAS4 - first "freak" head: round face (Rei's grandpa, who needs a child body for height)
CAS9 - second "freak" head: snub-nosed baby face (baby Koenma)
CASA - the adult version, shrunk, for Toguro Ani
CASB - the adult version, shrunk
The sample textures are more proof-of-concept than bases for editing. Based on the C002FC head, they have already been reduced to 256 colours (for the single heads, crop off the black part) and show the same eyes enlarged to different degrees to fit the socket sizes:
"cas8" fits adult and child CAS3/8, adult/child CAS9, child CAS5/0, child CAS2/7
"cas0" fits adult CAS5/0 and child CAS6/1
"cas7" fits adult CAS2/7
"cas1" fits adult CAS6/1/4
"cas4" fits child CAS4
"casa" fits adult and child CASA
"casba" fits adult and child CASB.