Was Hornblower a King’s Letter Boy or a Midshipman or were they the same thing?

It seems that in the first books written (so about fourth or so chronologically) Hornblower has always looked back at his time as a King’s Letter Boy. It is from my understanding that the system was abolished in 1729 way before Hornblower was even born so is it just another colloquial way of saying Midshipman? As I’m aware Hornblower held a warrant the second he started his naval career as he started quite late which is not the case for King’s Letter Boys.