I will try to look at my old "Railway Observer"s for that period, but as a first guess, it would be freight originating in the Wrexham area. They were too heavy to use on the Cambrian lines (and the line from Ruabon via Dolgellau had closed by then, anyway), and I don't think Croes Newydd had and regular steam-hauled passenger work at that time. Possibly, if any found their way to Chester or Shrewsbury, they might have been "borrowed" to work the Chester / Shrewsbury section of Birkenhead to Paddington services.
Sometimes, though, locos from depots due to close were transferred to surviving depots with space to "temporarily store" locos until they were needed elsewhere.