As I've tried to emphasise several times, this statement simply isn't correct for many of these 15 ton cranes. The nine purchased by the Midland had specially designed and constructed match wagons - in a previous post I linked to the drawing; their running numbers are known. The LTSR example also had a specially-constructed match wagon. No "conversion" involved. I'm not so well up on other companies' cranes but the Great Western match wagons were also, I believe, specially constructed.