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A PFC is as high as he could have gone in 2 years. NOT that I am a Swimmer Supporter - Them’s just the Army regs.

If that was true in the 1950s I know it sure wasn't true later during the Vietnam war. Are you sure about that?

Time in service, time in grade and MOS were the determining factors in enlisted rank, and to a lesser extent Occifer rank, advancement.
If a person had less than 2+ yrs time in service and they advanced beyond E-3, PFC, then it was a rare case of their having the MOS, usually Infantry, that allowed that to occur. It was done on the rare occasion to fill a slot that needed filled in a hurry and meant that no suitable replacement could be shuffled into the slot. OR the person being advanced possessed 'certain skills' needed for the slot.
In the mid-70s I knew of some enlistees getting credit for civilian skills that were needed in the Army who were brought in as PFC, advanced to Spc 4 after basic and after doing a series of MOS qualifier test. They then spent the remainder of their 1st year as Spec 4 and were advanced to Spec 5's after their 2nd year. But these were rare cases.
But then again, here we are speaking of a 'rare case' that did involve outside ..."encouragement."

When I was in the USN between 1975 and 1979, I made E5 and that was absbolutely as high as I could have gone in that time.

I know because I studied for the tests, and took them absolutely as soon as I was elegible, and made the first cut each time.

Unless the rules have changed (or unless they are different in a war zone or certain specialties, which is entirely possible) you couldn’t make E4 in two years.