The difficult part of RPN is often not arithmetic. It is knowing where a value is, when the stack will lift, and what a binary operator will consume. A screen can show those facts after a keypress; the StackCalc Learning Lab asks a learner to predict them first.
RPN calculators already exist. StackCalc is not a nostalgia project trying to recreate a familiar beige machine. The gap we see is a learning path: a way to move from a familiar screen to a physical instrument while understanding what the stack is doing.
That is why StackCalc joins the current iPhone and Apple Watch apps, a shared calculator core, Learning Lab prototypes, and an RP2350 handheld whose board is in fabrication around the same calculation behavior. The connection is semantic, not cosmetic.