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)

Ted, interesting '56 manifold.  My '56 manifold has flat carb mounting flanges, no cast depressions as your picture shows.  Also, the casting number on mine is on the bottom of the manifold on the water passage part of the casting.

John - "The Hoosier Hurricane"

Ted
I have been looking for that style of 56 manifold for a while now. Its only the second one that I have seen with the part# on the topside.Goes to prove that there were two different runs of the 56 manifolds

Like Hooser said most of them are cast with the part# on the bottom. If that manifold is for sale OR if you want to swap me for a bottom cast 56 style manifold - please let me know.
Thanks

Oldmics@Yahoo.com

Is there any performance difference between the 56 & 57 manifolds?
Thanks
Jim

A little bit, http://www.eatonbalancing.com/2014/01/17/y-block-ford-dual-quad-testing-on-aluminum-heads-part-ii/


Lawrenceville, GA

Prety sure I have one if you are interested.

Hi Slumlord
If you were referring to me re the manifold, I have one, but had no idea if there was a difference.
Now that I know my 57 type makes an additional 50+ Hp, I'm going to use it :-)
Thanks
Jim

I was refering to Oldcomics2. Unfortunately what I had was not the one he was looking for.

Got a question while we are discussing manifolds. I have a '56 manifold that has been modified by grinding out the divider so it looks like the kidney shape of the '57 manifold. I've been told that this has been apparently done a lot and signifantly lowers the price on these. How does affect the performance?

Here are pictures of a 1956 EDB manifold modified as you describe.  The four independent throttle holes have been converted to the kidney shaped holes as found on the 1957 ECG 2X4 intake manifolds.  This modified manifold was dynoed along with the other manifolds on an aluminum headed engine and it was down on power when compared to a stock EDB manifold.  Here’s the link again for the dyno test results.
 
Dual Quad Intake Testing

  

 

Lorena, Texas (South of Waco)