Popped down to the library to see what I could find about pencil stays. Found two references both from the Cycling Handbook by A.L.Pullen. 1.1950 Edition " Seat stays are now often made in the popular Pencil size, an introduction from the continent, but there is a point where elegant slimness becomes dangerous thinness and one should avoid specifying very thin stays because they look good. Especially on track machines and tourers where heavy loads are carried. One point to watch is the seat stay bridge which carries the back brake and is subjected to very heavy compression". 2. 1960 "Seat stays should always be in fairly stout section tube, pencil seat and chain-stays being insufficient".
Best wishes and be lucky. Michael Butler Huntingdon UK.