According to Day: “The illustration in the top right corner is probably intended for a Roman galley or perhaps for the ark.” The ark was also my first thought.
The ark seems to have been part of the third degree at some point as can be seen on a third degree “Masters Carpet” from the 1920’ies that can be found on the Phoenix Masonry website (scroll all the way down). If that were the case (a third degree symbol), it is up way too high on the scroll when we follow Day’s suggestion that the scroll is progressive in degrees from bottom to top.
For more information see here. Interestingly, the Ark seems to be appear on the oldest known symbol charts of ‘higher degrees’.