I think I got this data from one of the Mainline Modeler magazines:
Phase I..January 1945 to June 1947 Same as a F2..small combination numberboards/class lights; 3 portholes and no filters in the middle side course; tall fans; chicken wire screening in three pieces on the upper side course.
Phase II..June 1947 to June 1948 large numberboards; tall fans/countersunk low fans optional; two portholes, center porthole eliminated and substituted with four horizontally slotted air filters between them; larger openings at the rear of the upper course; chicken wire screening covering the upper course and the entire area between the portholes in the middle course.
Phase III..June 1948 to February 1949 large numberboards; Countersunk fans with grill type openings;two portholes with four horizontally slotted air filters sets between them;chicken wire covering the entire upper course only.
Phase IV..August 1948 to February 1949 same as Phase III with a fabricated horizontal stainless steel grill covering the upper course; two portholes with four horizontally slotted air filters sets between them; These were equipped with F7 traction motors and are sometimes referred to as F5's.
Here's SAL 4027:
and SAL 4030:
Fans and filters are the most prominent differences I see.
William McClure's "Atlantic Coast Line In Color" book has a beautiful photo of purple F3A 344 (phase IV) on page 34. On page 35 there's a color photo of F7A 393, I can't see any external differences between it and F3A 344, except maybe the horns and the color swap on the nose medallion.
