The first two members of the Merchant Navy class had their air - smoothed casings made of sheet steel .
21C6 was one of eight in which the casing was made of asbestos board , with a visible horizontal fixing strip along the centre line .
Although the first two members of the Merchant Navy class had their air - smoothed casings made of sheet steel , 21C6 was one of eight in which the casing was made of asbestos board , with a visible horizontal fixing strip along the centre line .