My Grandfather worked for the Fairey Aviation Company during WW2 in fact he was one of the Bosses, he had first hand experience of these pelts through test performed by the company, I remember him talking about them years ago. There not a lead replacement they do as stated previously act as a catalyst to obtain better combustion of the fuel, as also state this was due to the lack of refinement of the Russian grade aviation fuel, which was very poor in quality when compare to the British equivalent High octane high lead petrol, I remembering him tell me that the aircraft sent to Russia perform very badly on the home nation fuel in fact if caused internal mechanical damage to some Merlin engines, and meant others would not run at all. The main problems were at start up and even more so in cold Russian winter weather, the tin pelts help the situation but were in no way a cure, the real answer was RAF grade aviation fuel.
As to Tetraethyl lead being lethal, toxic etc, try this for size and sheep graze on this old WW2 site :eek:
Nige H