The Museum of HP Calculators
An algebraic scientific/statistics calculator. This model didn't have a four-level history stack but still made the previous result available via the LAST key and a SWAP key allowed the numbers surrounding an operator to be reversed. (eg 5/25 SWAP yielded 25/5.) The calculator had a single line dot matrix display that could display either a menu or a number. Other features included:
The calculator used the 26 lettered variables A-Z rather than numbered registers. While these variables were used in much the same way as numeric registers, they were more than just a relabeling. The lettered variables were not allocated from memory until used and they could be individually deallocated by simply storing 0 in them.
When RCL or STO was pressed, an A..Z annunciator appeared. The letters A-Z were printed to the sides of the keys. All the current variables could be scanned with the VAR command.