A total of 500 of these wagons were built between 1937 and 1959. The earlier wagons had four bolsters, coded MLE, while later wagons lacked bolsters but retained four stanchions on each side and were coded UME. Some later wagons had the bolsters or stanchions removed and were fitted with holding blocks for containers. These were coded FME for wagons fitted to carry LCL containers and GME for wagons fitted to carry 2 x 20'-00in ISO containers. Twelve had a centre mounted large swivel bolster for operating in pairs, or with spacer wagons, to carry l
The first group of wagons had diamond frame bogies when built, but these were replaced with plain bearing 2AP type, which became the standard. The cast drop centre 2BJ was later fitted to most, but not all, wagons. Later still the curve frame 2AQ with roller bearings became the main bogie for these wagons.