These bogie ballast wagons were a development of the 'Walrus' bogie hopper wagon which originated on the Southern Railway.

The model is based on a 'Sea Lion' that has beren converted into a 'Sea Cow'. The 'Sea Lion' and 'Sea Cow' were very similar in appearance but the former had two vacuum cylinders on one end and a single air cylinder only on the other end.  The 'Sea Cow' had the single air cylinder only but originally had a through vacuum pipe.  The name change reflects the removal of the vacuum cylinder. The hand wheel below the sole bars is a hand brake while the three hand wheels on the end platform were for controlling the ballast chutes which may be seen between the bogies.