I mean, we could do that, have the pre-war loco (No. 181 Brown Trout) be the first of the class, then get rebuilt into the post-war version (No. 4800 Brown Trout), and when Collet becomes CME he gives Grange frames to about half of the worn-out Churchward 4-6-0s, and rebuilds the other half with Castle boilers as an successful experiment.