No Valve cover gaskets ?????

[f.monroe]

Does anyone know when GM started using Silicone gasket maker instead of gaskets on the valve covers . The reason I asked is that I installed new cover gaskets on my 78 today and there were no gaskets , just orange silicone . Could this be the way they were originally installed or did someone do this over the years ? The valve covers did not have the slightest bend around the bolts, the original plug wire holders are all in place and not broken , the truck is very low mileage and was with the original owner until 2002 , and the silicone was way to set to be recently applied . It was almost hard and was a trick to get off . It was also a perfect width bead that squeezed over the rail on the head perfectly . Also what plug wires were originally installed ? This truck has some grey wires and they are stamped Delicore . All of the hoses are stamped GM and the tower clamps are still on . It looks like it was never touched under the hood . Frank