After WWII, farming demand was to have larger more powerful tractors, to cover the ground, however, these were only bought by the larger farms. Smaller farms needed to keep up, and Ferguson and Ford were producing a tractor to meet their demands. David Brown needed to keep up, and developed an alternative to the Cropmaster. The easy option was to produce small lower powered tractor, based on the David Brown Cropmaster. David Brown developed the 25hp and 30hp engine, and so the DB25 and DB30 came into existence, lasting from 1953-58. The petrol/TVO models were known as the David Brown 25C and 30C, while they called the diesel versions 25D and 30D.
Ref: Farm Classics, Volume One. David Brown Tractors 1936-1964 by Alan Earnshaw
More information to follow shortly, but please contact Stephen in the meantime.