For Goods Guard, Porter, Signalman, etc. This hat is British manufacture, in the correct railway BLACK colour (not navy blue), and has a real leather internal lining band (not plastic imitation). It has front eyelets at 2" centres to accommodate totem badge mounting.
Measure the circumference of your head in centimetres, just above the ears, where you want the hat to fit snugly. If you are halfway between sizes, go for the larger size, and fit some padding behind the inner leather band. Please do not assume that the size stated inside another hat you already have is actually correct.