LEGO Star Wars 77904 Nebulon B-Frigate was planned to be exclusive to San Diego Comic Con 2020, but the event was cancelled as a result of the global pandemic. Brickset has shared an image of the set, with the Brick Fan publishing three additional images.
The Brick Fan shares this quote, which reads like a product description:
Recreate every cool detail of the Nebulon-B Frigate with this brillant LEGO Star Wars construction set. Remember dramatic scenes aboard this Rebel cruiser as you build, then display your cretaion on its brick-built stand and show it off to friends. It’s the perfect way to celebrate the 40th anniversary of Star Wars: The Empire Strikes Back.
The first ever model outside of an Advent Calendar of the frigate uses 459 pieces, while the box features the special logo for the 40th anniversary of The Empire Strikes Back.