This picture on Geir Isene's website (linked in this website's links page):
shows a Halfnut. Note the rounded corners of the display. The Coconut's display corners are square. This is the first giveaway, since the insides are not immediately visible. The Halfnut started selling in '85, which would mean serial numbers starting with 25. Note that HP never had a "Fullnut" however. This is a name the HP-41 community uses to refer to the models before the Halfnut; but HP codenamed the original the "Coconut" (which came out in 1979); and then in the early planning stages of the next version (in 1983), there was talk of cutting the manufacturing cost in half by cutting down to two ports and cutting the IC count in half, so it was natural to give it the code name "Halfnut." They reduced the IC count more
than half but fortunately kept the four ports, and made a slightly better unit that's more reliable too. Many users won't notice any real difference.