Not trying to give you a hard time...I appreciate your posting...but, I would imagine sloppy would occur with lets say DMV Title Transfers where dozens are done on a single day. I would imagine, that with only two hospitals; and the population in HI not being that dense, that BC would be pretty rare (a couple dozen a week). So...if a sloppy mistakes would be a little harder to explain away (as opposed to the very busy DMV or Welfare Office).
Hospital paperwork mistakes are common. My Dad was in the hopital earlier this year and 3 paperwork mistakes were made in 4 days.
Don't jump on the gun regarding this number deal because no one here knows the procedures or lack of that created these BC number/date process. It's all speculation at this point.
I found data sugegsting about 18,000 births per year in Hawaii in 1960. Even discounting for births on the other islands, and for the army hospital, there were probably closer to a hundred births each week in the hospital. Oddly enough, one of the sources I consulted turned out to be written by the very woman whose twins started this thread:
The peopling of Hawaiʻi By Eleanor C. Nordyke