Dana Bell has teamed up with Classic Warships to produce a series of Aircraft Pictorials on U.S. Navy Airplanes. Three have been published so far, covering the SB2U, OS2U, and F4F. See http://www.classicwarships.com/aircraft_pictorial/aircraft_pictorial.html
These are affordable, well-researched, copiously illustrated monographs. The production quality is excellent. Dana has spent a lot of time on what I call an Easter egg hunt in the National Archives, among other places, and found pictures, drawings, and information previously unpublished and that sometimes contradicts or expands on what we know about a particular airplane type.
Dana is currently sifting through boxes and boxes of 1940s documentation on the F4U-1 Corsair. One of his discoveries is that the F4U-1A designation of what the Navy called the "raised cabin" Corsair was never officially assigned. (There are more revelations but those will have to wait until this next Aircraft Pictorial is published.)
His assertion has generated some debate. After looking at some of my stuff that dates back to 1943 and 1944, I agree with Dana that the Navy did not officially recognize the raised cabin F4U-1 with a designation change.
In the process, I got interested in the Navy's use of letter suffixes to aircraft designations. As a result, I'm pretty sure that the early (through say, 1942) and subsequent meanings have been inadvertently conflated in a confusing way. For example, in explanations in books and on line, you'll find that the letter suffix A stands for many different changes. After examining several examples and in particular relying on an explanation in Introduction to Naval Aviation (NavAer-80R-19) published in January 1946, it appears to me that up through about 1942 there were very few letter suffixes that were related to specific applications or capabilities. (One was P for Photographic.) "A" simply meant the first "minor modification not sufficiently important to change the modification number or where the airplane has been diverted to another service or for a special purpose".
It took a while to find an example of a "B" for a second minor modification that substantiated my thesis, which was the Consolidated NY trainer.
So in my opinion the suffix A originally didn't actually stand for amphibious (PBY-1A), armament (J2F-2A), target towing (JRF-1A), floats (TBD-1A), single-stage supercharger (F4F-3A), carrier-basing (SOC-3A), etc. It simply distinguished the modified airplane from the baseline one, like a dash number change signified a more significant change.
At some point early in World War II, however, the use of A appears to have acquired a specific connotation, possibly used only to differentiate a Navy airplane built for the Army, e.g. SB2C-1A. "B" similarly became associated with a Navy airplane that was to be provided to the British, as in F4F-4B. As a result, the first minor modification was now recognized with a "C" suffix and the second, with a "D". This led to the TBF-1C (wing guns and more fuel), the TBF-1D (ASB radar), SB2C-1C (cannon), F4U-1C (cannon), F4U-1D (multiple improvements), etc. Note that "C" did not, in this instance, stand for "cannon": it was a coincidence that the first minor change was to cannons. Thinking that D stood for droppable fuel tank in the case of the F4U-1D was even more fanciful; the F4U-1 had provisions for a droppable fuel tank.
Shortly after this philosophy change, however, mission-specific suffixes began to proliferate and the use of a generic suffix progression in alphabetic order beginning with C was abandoned. One of the early additions was "N" for a "night" fighter (as an indication of the designation system development in progress at the time, the radar-equipped fighter version of the F4U-1 was designated F4U-2; the equivalent F6F-3 modification was the F6F-3N.) But I can't yet say whether a particular suffix used in the mid-1940s was the result of the alphabetical approach or the implementation of specific letters for particular mission capability modifications. "C" was eventually used to designate a land-based airplane that had been adapted to be carrier capable but it's possible that the SNJ-3C was so designated because it was a first minor modification of this trainer. Similarly, "E" came to mean an electronic modification, but the TBF/M-1E might very well have simply been the third minor change of the Avenger.
Consistency in designations was not a Navy hallmark in any event. For example, the modification of the Douglas Devastator to add floats was designated TBD-1A. A similar evaluation of the Vought Vindicator was designated XSB2U-3...
*Consolidated Aircraft was merged with Vultee Aircraft in 1943 to become the Consolidated Vultee Aircraft Corporation. After the war, the company adopted the brand name Convair. It subsequently was acquired by General Dynamics but as a named division. In 1994, GD eventually sold that portion of the division still based in San Diego to McDonnell Douglas Aircraft and the remainder that had been developing and building airplanes in Fort Worth, Texas to Lockheed.