I know of the early HP-41C, with the "high top" keys, as some people call it. The slope on the front of the keys was sharper and, if I remember correctly, a bit harder to read than the "improved" version that came later. That later version had a more sloped key shape. You could read the blue legends on the front of those keys easier. At least that was the claim when HP released it. That version omitted the contacts in the port on the side that you would access the battery compartment from. Both those versions were "full nut" as evidenced by the squared off display.
Now, I see on e-bay a version of the HP-41C (not CV or CX) that has the rounded display bezel. How common is the "half nut" version of the HP-41C ?