The HP 15C was one of five calculators in the 10C series that were made from 1981 till 1989 (except for the 12C which is still being made after over 20 years). It's siblings are the 10C, 11C, 12C and 16C, all of which use RPN instead of algebraic entry. The 10C and 11C were both scientific calculators like the 15C, though not as powerful. The 12C is a financial calculator. The 16C was the first and only HP calculator designed for the computer scientist, with features like binary, and hexadecimal conversions along with many other functions useful to a computer scientist. At only 5" (127mm) long by 3.15" (80mm) wide by 0.6" (15mm) tall, the 10C series calculators are the smallest RPN calculators HP has made. This the perfect shirt pocket size. Battery life is long, it is not uncommon for a 10C series calculator to go years before needing new batteries. The 15C was the most advanced scientific calculator in the line. It was programmable, had built in support for complex numbers, matrix math, numeric integration, and root solving. Like most other HP calculators it had a very nice keyboard with a positive feel and easy to press keys. The layout was well thought, which made it easier to use. Unlike the calculators built today, the 15C has a simple elegant design. This design quality is, I believe, the reason the 12C is still being made today.
This is a nearly perfect example - the best I have seen. There is the option of the case in brown leather for 50€ more (instead of the original vinyl case)