I don't know. I've had two 283s...one in a 1957 and one in a 1958 and neither one used anything like that insert piece you have there. The exhaust pipe was joined directly to the manifold with a donut ring gasket on each side. Must have been something used in later years?

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end