@Dennis-LaRock posted:post war *steamers* 4 digits noted O27 specific locos... i.e., 2020, 2035, 2037, 2046 etc. This is not to say that some 3 digit steamers would not work on O27... just that it would not be optimal.
2020=671
2035=675
2037=637
2046=646
All identical equivalents. Not really a good rule of thumb.