That intake is very rare, it's a lab rat. The 73 AND 74 455 SD engines when designed were to have an aluminum intake manifold and that is what you have. The factory brass didn't even want the 73-74 SD engine. The production 73-74 SD engines were assembled with cast iron intakes. The aluminum SD intake was scraped because the brass was putting the heat on the engineering staff. I guess they did make a few but they never made production. You have a very rare intake manifold.
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