Hello…
This is my first time posting here. I was wondering if anyone knew that when the pilots and ATC call in or out for
a B747-8 they use the code word “74H” because there was already an aircraft named “748” ? 
The HS748 you are referring to I believe has a code of A748 (Avro was the original maker of the aircraft).
Was Avro also ATP ?
The ATP evolved from the HS748. It was made by British Aerospace.
The AV748 was first produced in 1960. In 1963 Avro became a subsidiary of Hawker Siddeley and the aircraft was renamed HS748. Hawker Siddeley became a founding component of the nationalized British Aerospace and the aircraft was once again renamed, this time as the BAe 748.
Thank you. Hence the B748 being called 74H
According to FlighAware, the 747-8 is coded B748. I didn’t see any aircraft marked as B74H.
Looking at the latest FAA contractions publication and amendment, I didn’t find either B748 or B74H as aircraft codes.
Yes, 74H is the IATA code for the B748 (which is an ICAO code). H is the 8th letter of the alphabet.