from this site:
The Unique Guitar Blog: The Gibson Firebird and The Gibson Thunderbird
The original models are known as
reverse Firebirds, since the body’s treble horn is longer than the bass horn.
(as most guitars AKA Fender - my input) Dietrich used the Explorer as a basis, but rounded the guitars edges into a more appealing shape. McCarty (who designed the Explorer & V) was the one who hired Ray Dietrich.
The Gibson Roman numeral designation was for the whole Firebird/Thunderbird line. As you can see, II and IV are missing, which were the two basses.
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