Rubber...well specifically neoprene hammer tips came about as a result of the major redesign that Fender undertook from 1969-1970. The original CBS/Rhodes design used felt hammer tips which were one of many expensive aspects to production Fender hoped to change. (The other notables being MUCH lighter tone bars and slimmer tines. As well as cheaper damper felt.) I often complain about Fender's cost-cutting - but the fact is that Fender managed to make the Rhodes cheap enough that they sold around 250,000 in total and that's why so many people can still play them today.
So the engineers at Fender came up with some small neoprene tips. Neoprene is basically a chlorinated polyolefin which was first synthesized in 1930. The cool thing about chlorinating is that the chlorine molecules "fill the gaps" per se in the carbon structure of the polyolefin making it denser, more impact resistant and more resistant to ozone/atmosphere. To imitate the hardness of the original felt hammers (which became progressively harder as the felt was reduced from around the wooden core) they manufactured these new neoprene tips in a 4 (well sort of 5) hardnesses. At first Fender had colorants added to the tips to make them easy to identify. However it turned out that the dye made the tips degrade very quickly so they switched to spraying the extruded strips of neoprene before they were cut.
Shape: Fender tried a number of shapes for these hammer tips. The first generation were sort of rounded like the felt hammers they were replacing. Then Fender switched to these square-type tips. They ultimately settled on a modified tear-drop style which was a little more precise about the striking surface and angle of attack. The neoprene tips, opposite of felt hammer, get slightly taller as they approach the treble section. The main concern being a clean let-off. The new thinner bass tines in particular move a great deal and a shorter hammer tip made it so they were less likely to touch the hammer tip after a strike. Whereas the treble end has very limited space where a sharper, taller triangle was necessary.
Hardness: We talk with a lot of techs and it's rare that I've talked to anyone that's perfectly happy with the voicing across a board. In an ideal world we would have twenty or thirty sizes and hardnesses of hammer tip, closer replicating the gradual attack of the original felt. But this is the real world people! So Fender decided 5 hardness levels was enough and extruded the softer 4 in long strips and then had them cut on an auto-feeder. The 5th, the wooden core will get it's own paragraph.
Hard-core: Because of the constraints of both space and hardness neoprene couldn't quite fit the bill for the highest treble end. However wood alone could damage the tines and also sound too different from the rest of the hammer tips. So a hybrid tip was born. Originally wrapped in the 3 layers of neoprene heat shrink, the design and materials also evolved along with the rest of the hammer tips. The size of the wooden core, the number of layers and the thickness of those layers has changed over the generations and re-designs. Fender had the advantage of large scale production but today these tips are made quite slowly by a handful of makers like Avion Studios. We lose a little less than 1/4 of the raw material during production - making these tips about 5X more expensive to produce than the others.
Special note - some customers have brought to our attention that some makers have used oak instead of maple. Oak is about 2/3 as hard as maple and also harder to mill properly.
Conclusion: I hope this has been at least a bit useful or interesting. Mechanical keyboards often live in a really fun junction between engineering and imagination. These odd little hammer tips represent a creative solution to a unique problem. Like a lot of what Fender did, they aren't perfect but they are functional and relatively inexpensive. It was fun for us to get into the engineering and production aspect of this small piece to the Fender Rhodes puzzle. Please let us know if you have any questions. We're here to help!