Interesting speculation.
If there is an error somewhere, I also notice the same has happened with another of JMW Turner's watercolour pieces.
'Newbury, from Speen Hill' circa 1805 is listed on the Tate Collection here:
http://www.tate.org.uk/servlet/ViewWork?cg...p;searchid=9289It is also attributed to a 'William Turner (of Oxford)', having also been engraved by William Byrne here:
http://www.gac.culture.gov.uk/work.aspx?obj=20238Interestingly, both this and the Donnington Castle lot were purchased at Sotheby's 22nd February 1977...