Almost
none of it is. ABS is one of the most common plastics, but
you'll find others, including PVC, glass-filled nylon,
polypropylene, HDPE, and more.
There are some filled plastics used during the 90's (probably ABS)
with what seems to be a too-high proportion of filler. These tend to
be almost fragile now, shattering into many small pieces when
subjected to any force at all. I've resorted to fiberglass mesh and
polyester resin to repair and reinforce these, finishing up with a
coat of paint. Doesn't look bad, but is hardly original.
I played with the idea of making a mold from the old pieces and using
acrylic resin to cast a duplicate, but there were too many details
(mounting bosses, ribs, tabs, etc.) on both sides to assure success.
I have rapid-prototyping equipment here, which is capable of creating
small parts like those. The only problem is that I currently have no
way of scanning anything in 3D. I have to take measurements and
manually generate a CAD drawing. It makes it less than convenient.
Peace... Sridhar