The one on my 442 is stamped with a 1974 date on the bottom.

 

I've seen a couple of the '70 W31 intakes with '74 casting dates on them,but that's the latest I've seen. There was one on epay not too long ago that was NOS still in the GM box with a '74 date on it. I think the guy wanted $2700 for it! I'm sure they sold a few over the parts counters once Olds nuts saw them on the '70s I'd bet '74 was the last production run for them. I wonder if the Winters foundry still has any production records???