What's the deal with exhaust manifolds having the same part, with or w/o A.I.R. smog?

[71CHEVYSHORTBED402]

I have a 1971 BBC, but the same issue may apply to SBC. At any rate:

According to GM's parts list, printed 1979, 3994045 LH is not for A.I.R (i.e., smog). But if you look online, you'll find the 3994045 part number on manifolds with the four port holes while others are without the four port holes. What the heck, same part number for both? My manifold is without.

Same issue with the 353030 RH, only vise versa. Per the book these are supposed to be for A.I.R. Some have the four port holes while others don't. My manifold is without. That said, what's the small hole for located on the 353030?..... For all I know it's just on some of them. It's located right above the "RH" mark. See the picture below for a reference. Maybe it's to mount a heat shield? The hole is plugged up on mine.

Note: I don't have A.I.R. I believe they were put to use in 1972. I imagine it varied by state.

Incidentally, there's little information on the web for 333846 RH ("not for A.I.R.") and 343787 LH ("for A.I.R."). The 3994045 LH ("not for A.I.R.) and 353030 ("for A.I.R.) are far more common, which of course are the one's I have.




Another reference. GM's parts book, 1979: http://www.gmpartswiki.com/getpage?pageid=21508