It wasn't actually a Gresley design although he bore the responsibility (and no doubt insisted on 3 cylinders with his conjugated gear and the general outline being compatible with the 'Gresley Look'. ) . They were designed by NBL. And depending on what drawings are available to the replica builders one needs to bear in mind two things - the first batch of frames were insufficiently strong and had to be replaced in short order and also the new build will not be subject to the drastic weight restrictions of the GER. One wonders if the civil engineers were ever told about the stronger replacement frames.