Powell Duffryn built 28 tank wagons for Bitumen in 1961 as an early Monobloc design. They were hired to SMBP, and were the first vacuum braked wagons in their fleet. They also had their fleet number. Early they had Shelphalt and Mexphalt name boards (see page 348 in Fidczuk, Peter & Bartlett, Paul W. (1991) Tank wagons, part 5. Early Monobloc tank wagons, Model Railways vol. 8 (part 7) pp 346 - 353). They became part of the SUKO fleet, but it is not clear how much use they had during the 1980s, when they were stored OOU. These had auxilliary suspension.