There is an autobiography by Walter Musciano that lists all of his articles, books, plans, designs, or whatever he has done (literally hundreds of items). There is a "Custom Special" listed as a profile Scientific kit for .09 to .19 power.
In another article written by Walt Musciano about Scientific Models, Inc, Musciano writes:
"By 1972, the forty-three history of Scientific model airplanes came to and end due to lagging sales."
So, I guess it would be save to say that the Musciano "Custom Special" appeared well befor 1976
Keith