I have an
HP-55 that's in really great shape.
Too lazy to take pictures but here's what I found out about the keys from
here:
"Based on the way that most people poked at the keys he designed a hinged key and avoided the 'oil can feel' of TI and other calculators. It took many design iterations to get the final design which used bent beryllium copper strips. The key bottoms were designed to be easy on the copper while still providing the right feel.
The keys were made with a state-of-the-art double shot injection molding technique. This was very difficult but ensured that key legends would never wear off."
On the keys I can very distinctly feel the edges of the labels. They are very slightly lower than the surface of the key, like infilled engraved legends.
About 2/3 down this page there is another description with pictures of the circuit board and metal strips(switches):
http://persistent-tech.blogspot.com/