I’ll take a swing at that ball, if you please.
During WWII and up until the early 80’s the U.S. Army essentially had two parallel enlisted ranks; the “normal” ones and the “specialists.” Specialists were typically in technical and support fields, whereas the “normal” ranks were typically in combat arms.
The Specialist ranks began at E-4, just as the non-com ranks did. Thus, and E-4 in infantry was a corporal, whereas an E-4 in commo was a “Specialist 4.” An Infantry E-5 was a Sergeant, whereas a Food Service Specialist was a “Specialist 5.” IIRC, the Specialist ranks went all the way up to E-9, though the highest I ever saw (in older photos, not personally) was a Specialist 8, equivalent grade-wise to a Master Sergeant/First Sergeant.
The Army began doing away with that in the early/mid 80s. My first duty station in Bamberg, Germany (July 86), had unit photos posted in the barracks, and the one for 1985, taken the previous October, still had a few remaining Specialist 5s in the photos, but they had since been converted to Sergeants by the time I had arrived.
The only remaining holdover from the old Specialist sytem of enlisted grades was the Specialist 4, later condensed just to “Specialist.” The major distinction (in Armor, at least, though I saw similar practices in Cav Scouts, Infantry, Artillery, and Aviation) was that Corporals tended to be in leadership positions, or positions of slightly greater responsibility than the equivalent graded Specialists.
Case in point: I left Germany as Specialist 4, on the E-5/Sergeant promotion list. At Ft. Hood, I was made gunner on our Platoon Leader’s tank, and was “laterally promoted” (there’s an Army euphamism for ya!) to Corporal. The Platoon Sergeant’s (that’s a title, not a rank) gunner was likewise a Corporal. But the gunners on the two “Wing Tanks” were Specialists.
While we Corporals nominally had more authority than Specialist 4s, it was a very fine line between the two; what it meant more often than not was that the “leadership” of the really crappy work and guard details got fobbed off on us, and the Specialist 4 (just called Spec-4s, as in “Speck Fours”) kicked back and acted all sarky about being bossed around, sometimes by guys with less time in service/time in grade than them.
It was an accepted truism amongst Corporals that we were really just low-budget Sergeants; good enough to do a Sergeant’s job, but not receive a Sergeant’s pay. That attitude, of course, changed remarkably once we received our third stripe. We suddenly understood why the Sergeant was so superior to the lowly Corporal, but were too cool to pass that along to our junior bretheren. 