I remember them from years ago too. They do work well, as long as it isn't too windy, which, of course, does often go with rain. We used to carry them rolled up round a spare tube secured to the back of the Brooks saddle with a spare leather toe strap.
That was in those funny old days when we used to have plastic strips round our wheels called mudguards. Modern fabrics for breathable coats have replaced all of it.