Kevin,
Although this may not satisfy your cousin, I think it is fairly accurate. Anyway, her e goes.
Source: "Sixty Years of Chevrolet" by George H. Dammann shows the following:
1942: Master Deluxe and Special Deluxe.
1946: Stylemaster, Fleetmaster and Fleetline the latter being a sub-series of the Fleetmaster, i.e., Aerosedan with its extra trim.
1947: Ditto except added Fleetline Sportmaster sedan
1948: Ditto
(Note: It seems that Fleetline merely indicated the extra trim etc. I think people nowadays may forget that starting in 1947 the term "Fleetline" was correctly applied to the some Sportsedans as well as to the all Aerosedans.
The above was taken from the "Revised Edition" of "Sixty Years of Chevrolet." The revised should have taken care of many errors although, maybe, not all of them.
Mr. Dammann expressed thanks to many sources. Among them was former VCCA president and editor of G&D, Duane Steele. Accordingly, I have better faith to the information than would otherwise be the case.
I noted that Gene had some information of a transition period in 1942. Owing to his vast knowledge about things Chevrolet, I think he is probably correct but the source book didn't mention that.
The book shows a 48 "Woody" Aerosedan but discounts it ever being in regular production owing to the fact that it was a prototype that failed to impress management. It was a pretty thing indeed. Too bad.
Charlie
BTW: I expect that there will be additional information about this instant issue of Chevrolet names for the 42-48 years. That's fine. I enjoy learning more and more of this kind of information. Please let me know if you find that any of the information attributed to the source was incorrect. Thanks.