Next to Texas Instruments, my favorite calculators are probably from National Semiconductor.  Many of these were marketed under the Novus brand name, which was adopted by NSC for its calculator division.

Novus Mathbox 650: My first calculator (01977) was a Novus Mathbox like the one on the left above.  A couple of different case styles exist; all have a decimal point fixed at two places which allows them to get along with only fifteen keys.  The Mathbox uses reverse Polish notation, something of a rarity on four-function calculators.

Novus Professional 822

Novus Executive: This unusual calculator is shown with its power source, a Duracell Flat-Pak 9-volt battery.  The battery is inserted in a slot beneath the silver strip at the right..  The Executive has exposed electronics on the back--the whole thing is designed to be carried in a folding case.

Mathematician 4510: Two name variations of this calculator exist, one under Novus and the other under National Semiconductor.  A variety of case styles was also produced--the first and third above have white cases; the second is black.  (A third, programmable, version, the Mathematician 4515, was also made.)  The Mathematicians contain an unusual "M + x²" key that is useful in statistical calculations.

Financier 6020, Financier PR 6025: The PR adds programmability to the 6020--both handle common financial calculations with preprogrammed keys.

1. NSC International Computer: Preprogrammed with common metric conversions, this calculator debuted in the 01970's at the height of America's failed rush to convert completely to metric measurement.
2. NSC 4660: A scientific calculator with a collection of unit conversion keys.