View this artwork by appointment, at Tate Britain's Prints and Drawings Rooms
This rather faint, slight sketch is difficult to make out, but does appear to how a man in armour, as Finberg suggested.1 There seem to be a hanging or curtain at the left and shadows beyond the figure to the right. The horizontal line about a fifth of the way up from the bottom of the page frames the image, and may indicate that Turner was copying an existing source such as a painting or engraving.
Finberg 1909, II, p.741.
The page is somewhat stained by proximity to the leather overlaps from the front cover and the acidic board under the front pastedown opposite (D41105).