This week we have sold an original HP15c in bos, and in pristine shape. While I was reviewing its compact box, the instructive manual and the machine itself, I kept thinking that it was the best hp product ever.
Capable of inverting an 8x8 matrix, solving equation systems with complex numbers, and having all keys with three different functions (and in some cases 4), it has the most elegant logic of all units I have seen. Nothing is superfluous, nothing repeated, everything has a purpose. And it is compact and beautiful - the ideal lab calculator.
Here are some pictures of it: