Can anybody help me with an approximate time Ford changed the rocker gear in the EL 4.0. EF rocker gear used straight valve springs which were carried on into the EL S1's Some S2's also had EF springs but later change to a cone type spring. My question is .... When???

I've been told the geometry is different between both trains and only one of the types (EF/EL valve trains) can be substituted and used for both.