I disagree. These sound like the LMS Roadstone wagons, of which there were 101 built in 1946-8. See pages 175 - 6 in Essery, R, J, (1981) An illustrated history of LMS Wagons, Volume 1, Oxford Publishing Co. SBN 86093 127 7. viii + 180 pages (which is back in print).
Paul
York