I love RPN calculators and collect HPs. I made this app over a weekend some years ago when the Windows App Store first opened. I haven't touched it since! I figured I may as well put the source out there in case anyone wants to do anything with it.
It is essentially a GUI for the HP-35 emulator I made at one point. It's an amazingly tiny F# "engine" running the original ROM from the 1972 device.
Have fun!