How do identify what year my engine is?

George Jets said
I would say 1976 360 by the looks of the plastic plug in the end of the valve cover.

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It could look like this pictured below. 1976 W200 360

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Something to compare your engine too.

Mine doesn’t have any plastic plugs on the valve covers that I can see. The thing you were seeing is the rubber grommet where there is a pcv valve in the middle of the passenger side valve cover. Here’s a photo I had of the drivers side valve cover.