My 3278 seems to be solid POM on the engraved caps and ABS doubleshot on the ABS caps while the Selectric II caps seem to be ABS shells doubleshot with POM for the white as well as some ABS/ABS doubleshots. There are extra tiny anchors in the ABS/POM to help hold in the notoriously difficult to bond POM. Turning them in the light, you can even see the separation of the two plastics in the top of the key. It doesn't blend into a single surface like doubleshot ABS.
The pictures aren't great but here's some examples of stuff.
Doubleshots. Yellowed ABS in center. Notice the little extra anchors on the POM stuff. One uses the walls of the cap and one uses the top.
Oddball mounts and more anchor examples.
What fits on the board. Notice you'll need two sets of keys to fully replace everything since you're short one standard key on every row.
