The D-30 was the first production model to roll out with the dorsal fin. Whether or not some of the D-25s were retrofitted on the field to have them still doesn't change the fact that rolled out of the factory, the D-25 did not have the dorsal fin.
Another example, some P-51B/Cs incorporated some of the improvements of the P-51D model, namely the repositioning of the machine guns in the wing to fix the jamming issues with the B/C models. These changes were done on the field and not rolled out of the factory this way. Same with the dorsal fin on the D-25 Jug.
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