Norm, I know it is hard to believe and as ludicrous as it may sound, but that fine Marque of British Mechanical engineering had to make cut backs during the war years arms build up and delivery period.
The taps were originally of high tensile drop-forged lathe-turned annealed Sintered Bronze. Unfortunately the supply of drop forged annealed Sintered Bronze was needed more so for the manufacture of Supermarine Spitfire Mk.XIV Aviation Fuel tank cap hinges and accompanying split pins.
Spagthorpe Engineering Dept during that time adapted to the shortage by using what was thought to be High-Density Nylon Polycarbonate heat resistant material to manufacture the above mentioned Blinker Fluid taps.
After the war and the downturn in the need for Supermarine Spitfire fuel caps became apparent, much to the chagrin of the head Engineer at Spagthorpe they discovered that during the war years they had actually been manufacturing the taps from recycled compacted British Army Hobnail Boot lace packets. So as to avoid an embarrassing faux par they continued this practice until 1952.
