The standard post-war gearbox, at least all the standard "BA" variants I've seen, had top-hat bushes on the layshafts, cracking of the case was not such a problem with ordinary road use. The Norton/A.M.C. gearboxes seem a little more prone to this weakness, especially with the standard drive side layshaft bearing fitted. I've got to admit, though that the 61207 bearing was a sublime drawing- office dodge!