Several times in its history, the Army has made a distinction between Non-Commissioned Officers, who have leadership responsibilites and authority over lower ranked soldiers, and technicians, who are technical experts without leadership authority.
In 1942, the Army instituted the Technician ranks. The reasoning was that men with specialized training should get more pay, but not necessarily the responsibility of an NCO. They were Technician Third Grade, abbreviated Tec3 or T/3, Technician Fourth Grade (Tec4 or T/4), and Technician Fifth Grade (Tec5 or T/5). Since radio operators didn't have leadership responsibilities, they were usually Technicians.
Rank and pay grade are two different things. Sergeants were senior to Tec4s, but they were in the same pay grade.
The Technician ranks were discontinued on 1 August 1948.
A quick answer to your question is yes. When your grandfather enlisted, he took aptitude tests whose results determined to which Military Occupation Specialty (MOS) he would be assigned. There were several radio operator MOS’s in the 750, 760, and 770 series, depending upon the type of radio equipment involved. The technical skills involved with these MOS’s brought promotion to the three Technician grades, T-3, T-4, and T-5.
An earlier History Hub discussion about this topic can be found here:
I Hope you find this information useful.