The HP41 model moved from Full-nut (separated keyboard and processor) to half-nut (unified circuit for keyboard and processor) around 1985. This increased reliability (very few half-nut require repair beyond the ubiquitous battery connector), but many felt that the screen was not as good as the previous model.
In 1985, there was no point in producing the C model: it was too limited and integration made easier to produce just the CV model, with 4 times the memory included. Thus, I had not seen any C model in half-nut guise - until this week.
Also the back of the calculator belongs to a much earlier calculator. We think that it may have been an official HP repair over an old calculator, where the back side was reused and a hal-nut frontside with C innards was fitted.
The calculator is now in the hands of a famous collector.