It has a Video generator, UART and the equiv of an 8048 inside, plus various glue logic internally and is designed as a Video Terminal chip, more or less like a VT100 type of thing, video terminals (perhaps inside other equipment) is the type of equipment it would be in - anything with a video terminal potentially. With me it's on what appears to be an S100 (or similar) type of board in a piece of industrial equipment, though it's not used in an S100 configuration and I suspect its perhaps mid-late 80's.
If it's possible to put attachements here (I can't see a way), then I could put up the first page of the National Datasheet, but alternatively the datasheet is available here http://www.datasheetarchive.com/dl/Scans-063/DSA2IH00122424.pdf
The Datasheet says -The NS405 is a CRT Terminal Controller on a Chip. It is a Microcomputer systems which replaces the following LSI circuits commonly found in a CRT Data Terminal:
-Parallel I/O Controller
The NS 405 is Primarily intended for use in low-cost terminals, but contains many features which makes it a superior building block for "smart" terminals and word processing systems...