is a calculator
very similar to the HP-20S
except that it is lacking in HEX, DEC, BIN and unit conversions and other features to offer advanced statistics
features and random number generator. It also has six different programs to load into memory as opposed to the six available on the 20S. The 21S had a higher list price, but didn't sell as well, than the 20S.
The 21S was mainly distributed in batches to classrooms, as each batch only had one manual, they are quite rare and certainly more valuable than the calculator to collectors.