HP-28S Advanced Scientific Calculator

Very long ago, when I was in high school I got my first hp calculator from my dad: a HP-15C. He gave it to me because I had issues with brackets on a standard calculator and with RPN you don’t need brackets. I believe this was in third grade. When things got a bit more advanced I got an HP-28S which was a huge step forward in user friendliness. The HP-15C only has 1 line on the display while the HP-28S has 4 lines and a stack which you can manipulate visually on the screen, like swapping the numbers in position 1 and 2 in the stack, dropping the bottom number and so on. The 15C has some of this functionality I am sure but you don’t see what is happening. Anyway, I really loved my HP-28S but at some point it died on my and I moved on to an HP-42 I believe and an HP-50G in the end. This last one I still have and this is my go to calculator if I have the choice. A few weeks ago I was surfing the web and came across an offer for a second hand HP-28S on eBay and bought it. It just arrived and is working (had to go out and get a set of batteries). Inserting the batteries and closing the battery door had me struggling a bit, this is definitely a poor design. The clamshell with keys on both sides is unique as far as I know and allows for large keys with large print on them and less need for shift keys to access additional functions on the same key. This is a calculator for the elderly with poor vision but without sacrificing on the functionality like what you typically see with mobile phones. That is of course because this is an old product, not designed with the elderly in mind. I wish someone would come with a new calculator with this clamshell design.