Broadly every major carriage works, and an equal number of the private builders, seem to have built BR Mk 1 stock in pretty much the same timeframe to within a year or two. Before about 1951-2 they were still building company designs (often still with wooden framing, ostensibly to use up seasoned timber stocks efficiently, and to provide work for long-serving carpenters, the safety aspects of a greenhouse-on-wheels be blowed), and after about 1963-4 Beeching had sussed that this number of workshops was grossly inefficient (there was a specific part of his study about workshops), hence the Mk 2 that came along at that point was wholly Derby built, and EMUs were concentrated at York.
