Mamod short safety valve (1/4 inch BSF) 
Short (approximately 1") ¼" BSF Safety Valve for early traction engines / rollers / steam wagons and some stationary engines.
Does not fit the older SE1, SE2 and SE3 which needs the wider bodied valve.
Will fit some SE1, SE2 models with small 1/4" thread, but not SE3 as the engine only ever had a 3/8th valve fitted.