Just out of curiosity, why do you use "G-39" or "G-38" at all? Those are typically considered just "design" numbers, not officially "model" numbers. In any event, the FAA recognizes only the model designation "G-21A" and all ex-military Gooses that have been re-certified and registered as civilian aircraft are recognized as such. I realize that the original civilian versions of the Goose (actual models G-21 and G-21A) were in the minority in terms of overall production numbers, but the fact of the matter is that all ex-military models and even the original 12 plain "G-21" aircraft were all supposedly brought up to G-21A standards for civilian registration in the US - if they lasted long enough. Of course, the most numerous examples of any Goose were the US Navy's JRF-5s, accounting for 184 out of the total production of 345.
Goose production actually broke down as follows:
12 models G-21 (all supposedly brought up to full G-21A standards)
30 models G-21A
1 model G-26 (US Navy XJ3F-1 prototype for later JRF series)
26 models G-31 (USAAC OA-9)
12 models G-21B (pure flying boats built for Portugal)
204 models G-38 (five JRF-1, five JRF-1A, ten JRF-4, and 184 JRF-5 for the USN)
10 models G-39 (seven JRF-2 and three JRF-3 for the USCG) and
50 JRF-6B* contracted for the British (although not all were delivered to them)
*Most reference sources call the British JRF-6B aircraft Grumman “G-38” aircraft as well, just like all of the USN aircraft, but if they were similar to any of them, it was only to the first five JRF-1 because all of the rest of the USN aircraft had camera wells (almost 3 ft diameter holes) in their bellies but the British JRF-6Bs did not.
So, defaulting to "G-39" just because it was the last or highest “design” number is not correct in almost every case and in every case, the FAA recognizes “G-21” and “G-21A” as the only actual, certified “models” of Grumman Goose aircraft. In fact, only two "G-39" aircraft still show up in any current civilian registry; ex-JRF-2 serial no. 1077 is still flying in Canada as C-FUAZ for Pacific Coastal Airlines in British Columbia and ex-JRF-3 serial no. 1085 (last registered as N12CS) is now hanging from the ceiling of the National Museum of Naval Aviation in Pensacola, FL.
We had to pick something and last usually isn't too bad. I've updated G21, G21M, and G21T to reflect the most generic option available:
G21 Grumman Goose
G21M McKinnon Goose
G21T McKinnon Turbo Goose
When the owner of N77AQ, for example, files a flight plan identifying it as a "G21/R" first of all, it should be identified as a “G21T/R”, but secondly it is still incorrect for FlightAware to label it as a "Grumman G-39". Although it is now a McKinnon G-21G (having been re-certified under a completely different type certificate, TC 4A24), prior to its turbine conversion in 1969, it was an ex-USN JRF-5 (s/n B-62) and as such it was originally a Grumman model G-38.
On the flight page we display the aircraft code the pilot files and the best name we have for that code. N77AQ now reads "Grumman Goose (twin-piston) (G21/R" since the pilot chooses to file G21. I realize the pilot is filing incorrectly, but we're not going to change it. The registration page says 1969 MCKINNON G21G since that's what the FAA registry database says.