The gaiters (leg guards) issued to the British Expeditionary Force (B.E.F.) beginning in 1937 were designed specifically for the uniforms introduced at the time. They differed slightly from the versions used during later wars.

They were designed to protect the top of the boots and the bottom of the trouser legs from dirt, mud, and moisture, and to maintain a neat appearance.

The gaiters issued to the B.E.F from 1937  were slightly different from later wars versions.

Brass tips on end of straps
No leather reinforced panel on inside
Size measured across the top, measure around your calf whilst wearing the clothing you will wear the gaiter around and select your size:

Size           Length
Size 3 = smallest  30cms longest 33cms
Size 4 = smallest 33cms longest 36cms
Size 5 = smallest 36cms longest 39cms