Computers of the old days and, especially programmable pocket
calculators, were not just mass-produced, noname PC compatibles like
nowadays, but possessed a personal character. Whenever you met
someone with the same calculator, heavy insider information
exchange was the consequence. You knew every bit personally, you
were proud of every single byte and tact cycle you optimized away.
It was always a highlight to find an undocumented feature! (One of
the most amazing commands is one which doubles the processor
frequency of HP28S). The date in front of the model is the buying date.