Jon,
My copy of the Douce sheet is not a very good print off, but the writing looks early 19th century to me, so probably written on by one of the collectors. You can see the copy at the Bodl. Douce 1.72. The writer's source for this information might be around somewhere. These 17thc Black-letter sheets have long commanded a high price even since the 18thc so I would say a collector like Douce wouldn't have written on one in ink and defaced it, but that's just an opinion.