I have noticed that the bottom surface of the factory dual quad manifolds sometimes has two large set screw type bolts while some manifolds have a completely smooth bottom surface. What is the significance of these differences? Pictured below is a two-bolt type.

 

Steve

Here are pics of the 1956 (top pic) and 1957 (lower pic) manifolds.  The most obvious difference is at the carburetor flange.  ’56 manifolds have four holes while the ’57 manifolds have the kidney shaped openings.
 

 
 
Here’s an updated list with Slumlords manifolds added to the list. Looks like 49 manifolds not counting the easyissy manifold that the date was unclear on.  It’s not on the list yet.
 
10-15-56 (JH)
 
12-5 (JH & RH), 12-6 (RF & Marc), 12-13 (JH), 12-14 (EP & Ted), 12-17 (TD), 12-21 (Ted), 12-28 (JH)
 
1-3 (JH from Ebay), 1-9 (DK & NoShortcuts), 1-11 (RG), 1-18 (JH), 1-22 (Genuinerod & Ted)
 
2-4 (JH), 2-7 (JH from Ebay & SL444), 2-18 (JH & SL444), 2-28 (EP & JH),
 
3-6 (JH), 3-7 (JH), 3-8 (JH), 3-13 (EP & JH), 3-26 (Mike), 3-29 (EP & JH & Ted)
 
4-3 (JH), 4-11 (SL444)
 
5-8 (JH from Ebay), 5-10 (EBay), 5-28-57 (fakirone)
 
6-3 (JH), 6-5-57 (Ted), 6-6-57 (Flatrod & JH), 6-10 (JH), 6-11 (Robs36 &Robs36)
 
No date (CL & EBay & SL44) 

Lorena, Texas (South of Waco)