Peter wrote Marcus; Not exactly sure, but I was under the impression it was mostly for shorter time trial. Mick, Norris or someone from the U.K. would probably be best at providing the answer
As a former time trialist here in the UK I confirm they were used by the majority of riders racing in the 1950/60's especially if the weather forcast was rain/showers etc. They did protect the saddle area at the rear and saved the riders face from being sprayed from the front wheel. Another piece of equipment commonly be used in conjunction with these shorties was Carlton tyre savers nicknamed " flint catchers". Thus both giving partial protection of both rider and bike in inclement weather.
Doug Smith
North Dorset
UK