N150M
With apologies to Bill B. (aka "Mr. Widgeon") I've been wanting to do this same kind of thing with Gooses for a long time - and especially with the rare McKinnon conversions, not just "Grumman" Gooses still certified under ATC-654 as modified by McKinnon STCs, but the very few that were fully "converted" - not only heavily modified but also extensively rebuilt as "new" aircraft and completely re-certified as "McKinnon" G-21 series aircraft under FAA type certificate no. 4A24.
McKinnon's very first full Goose conversion was done using a former "British" model JRF-6B, Grumman OEM serial no. 1147. It had been built in June 1942 as part of a contract for 50 such aircraft for the Royal Navy (not the RAF as is commonly reported.) However, s/n 1147, which was initially assigned US Navy Bureau of Aeronautics serial or "Bu." no. 0203B and British serial no. BW800 in conjunction with the initial procurement contract (but BW800 was NTU and serial no. FP497 later assigned instead) was actually held back from the British order and it was never delivered to them. Instead, it was handed over for use by the US Army Air Corps to help to make up for some of their losses of the same type (actually model OA-9 aircraft) early in the US involvement in the war.
After the war, when s/n 1147 was returned to the US Navy for final disposition, it was assigned a new "Bu." no. - 66357 - and it was eventually transferred, in Dec. 1946, to the US Dept. of the Interior for use by the Fish & Wildlife Service in Alaska. At that point, it was registered for the first time as a civilian model G-21A - as NC709.
In January 1957, it was sold to McKinnon Enterprises Inc. of Sandy, OR (USA) and "de-registered" so that FWS could retain the use of "N709" because all 700 series aircraft registration numbers were reserved for its exclusive use. (Its "sister" agency the Bureau of Land Management, aka BLM, used 600 series registration numbers and its Gooses included N640, N641, N642, N643, and N644, Grumman serial nos. B-123, B-115, B-137, B-145, and B-130 respectively.)
Between January 1957 and December 1958, McKinnon converted Grumman G-21A s/n 1147 into McKinnon G-21C s/n 1201 in accordance with TC 4A24 Section I and McKinnon Master Drawing List no. 7. Incorporated into it were many of the same features that had been previously developed for use on legacy Grumman G-21A series aircraft and approved by the FAA as STCs in their own right - features such as the extended, fiberglass "radar" nose cone, the one-piece "wrap-around" windshield, the enlarged main cabin "picture" windows, the retractable wingtip floats, wing leading edge landing lights, and the extended dorsal fin. Other features not covered by separate STCs for use on legacy Grumman G-21A aircraft were the installations of the four 340 hp Lycoming GSO-480-B2D6 geared, supercharged, opposed (i.e. "flat") piston engines and numerous internal structural reinforcements which most significantly allowed the model G-21C to be operated and certified essentially all the way up to the FAA's "Small" aircraft and single-pilot weight limit of 12,500 lbs.

This is McKinnon G-21C s/n 1201, N150M, in Bethpage, NY in April 1960 when Angus McKinnon went there to buy
1939 Grumman G-21A s/n 1054, N704A, from the Grumman factory, which had been using it as a support aircraft
ever since buying it back from a private owner in Pennsylvania in 1948. BTW, Grumman G-21A s/n 1054 was
originally painted overall red with a white "lightning bolt" stripe on each side and a black lower hull up to the
"waterline" and it was registered as NC3055 for the Texas Oil Co. - aka Texaco. Does that sound familiar?