Once, the parliamentary chamber was brightly lit by two large hanging lamps. The lamps have now disappeared, giving a darker tonality to the painting.

There are other changes, too. In the left foreground of the work Sotheby’s is selling, for instance, a seated chimp on the front bench holds a banana curved downward; in the 2009 version the banana is turned up. Nearby, a piece of decorative carving is a significantly different shape and size in the two versions.

Sotheby’s online catalog description said that the painting was signed and dated 2009 and was the work that had appeared in the Bristol exhibition, and that the current owner had bought it from Banksy in 2011. The description did not make any mention of the differences, though it did say the work had a new title, “Devolved Parliament.”

The auction house acknowledged the differences on Wednesday after being asked about them by The New York Times. Sotheby’s said it was the same painting, but modified by Banksy since its original showing. The auction house added that it had been aware of the changes, but did not explain why they had not been mentioned in the catalog.