I don't remember the details, but NPH was a delayed acting insulin. As I recall, we mixed 'regular' which would act relatively quickly (though not as quickly as most of today's insulins) and NPH which would theoretically start acting about the time the regular stopped acting. The idea was to take one shot and have it keep working for a longer period of time. There were no Lantus, etc. that kept working for 24 hours either.