Actually not quite as simple as that : re-reading Platform 5's 'Departmental Coaching Stock, I see that "The DB97xxxx series was first used at the end of 1966 ..." ...... so everything must have gained a regional 'D' prefix before then - but I guess there were few BR standard vehicles departmentalised that early ! ( Though from an early Lot of Mk1s, W1012 was only about a dozen years old when converted.)