They were actually intended as cycling capes, and had elastics on the inside to put your hands / handlebars (not advised!) through, to prevent the cape billowing up in the wind. They came in (I seem to remember) 4 Sizes, and were occasionally worn for foot patrols by some officers.
I bought one new and unused (from "Lau(w)rence Corner", for our older readers!) when the Met cast them some 40 years ago, with the intention of wearing it as a "Special" on night duty .