Ove,
The 16467 model is an Impala convertible (non-SS). The total production of that model was 42,712 over 12 assembly plants. If every plant made the same average number of convertible Impalas, (which they did not) each plant would have produced about 3560 of them. There may have been a couple plants that did not produce the convertible body style at all. Also, Wilmington was one of the higher volume plants. Fremont and Southgate were fairly low volume plants. So, there were a lot of "wiggle" factors involved in trying to make production volume guesses. But the bottom line is that your body number of 5551 makes perfect sense to me as the number of body style 16467 made at the end of the '65 Wilmington production year.
Hope that makes sense....
Verne