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"

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, my accumulation of  1957 2x4 manifolds are dated 2-18,2-7, 4-1, and one undated but I need to check between the carbs because when I checked the T Bird Club restoration manual they said some were dated there. I also have a '56 intake with no date but also will check between the carbs. Interestingly the '56 manifold is marked ECG9424D rather than EDB9425C although the manual states that most '56 manifolds were so marked. 

Here’s an updated list for casting dates on the 1957 ECG-D 2X4 intake manifolds.   The latest addition is a 1-11 intake manifold that came off of an original E Code Thunderbird.  The manifold date on that one predates the engine assembly date by about a month.  That TBird was sitting out in the open air on an island off the coast of Washington for the last 51 years.  There are still no November intakes posted 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), 1-18 (JH), 1-22 (Genuinerod &Ted)
 
1-11 (RG)
 
2-4 (JH), 2-7 (JH from Ebay), 2-18 (JH), 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)
 
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)

Lorena, Texas (South of Waco)

Hmmm...

Current eBay listing, item number 171232339569

Another intake with no production date on top of the thermostat area!

Doesn't appear to have been altered.

NoShortcuts
a.k.a. Charlie Brown
near Syracuse, New York

Here’s an updated list. It's now 41 ECG-D 2X4 intake manifolds that do have dates. There are still no November intakes posted yet. It would appear that there are ~5,459 dual quad intakes remaining to document.

.

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), 1-18 (JH), 1-22 (Genuinerod &Ted)

.

2-4 (JH), 2-7 (JH from Ebay), 2-18 (JH), 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)

.

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),

Lorena, Texas (South of Waco)

Nice thread! Sorry to be in so late but I have 2 of these intakes and they BOTH have the date 6-11 AND the S stamped.:-)

36 Ford 3W Coupe, 68 Merc Cyclone GT FB, 76 Chev Camaro, 87 Civic Wagovan, 88 Caprice Classic Wagon

Here’s another updated list.  It's now 39 ECG-D 2X4 intake manifolds that do have dates.  There are still no November intakes posted yet.  It would appear that there are ~5,461 dual quad intakes remaining to document.

 

10-15-56 (JH)

 

12-5 (JH & RH), 12-6 (RF & Marc), 12-13 (JH), 12-14 (EP & Ted), 12-17 (TD), 12-28 (JH)

 

1-3 (JH from Ebay), 1-9 (DK & NoShortcuts), 1-18 (JH), 1-22 (Genuinerod & Ted)

 

2-4 (JH), 2-7 (JH from Ebay), 2-18 (JH), 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)

 

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)

 

No date (CL),

Lorena, Texas (South of Waco)

[quote]Oldmics (10/1/2012)

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[quote]Oldmics (8/26/2012)

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Here’s the updated list for the 34 ECG-D 2X4 intake manifolds that have dates.  No November intakes posted yet.    10-15-56 (JH)   12-5 (JH), 12-5 (RH), 12-6 (Marc), 12-13 (JH), 12-14 (Ted), 12-14 (EP), 12-17 (TD), 12-28 (JH)   1-3 (JH from eBay),1-9 (DK), 1-9 (NoShortcuts), 1-18 (JH), 1-22 (Ted), 1-22 (Genuinerod)   2-4 (JH), 2-7 (JH from eBay), 2-18 (JH), 2-28 (JH), 2-28 (EP)   3-6 (JH), 3-7 (JH), 3-8 (JH), 3-13 (JH), 3-13 (EP), 3-26 (Mike), 3-29 (Ted), 3-29 (EP & JH)   5-8 (JH from eBay),5-28-57 (fakirone)   4-3 (JH)   6-3 (JH), 6-5-57 (Ted), 6-6-57 (JH), 6-6-57 (Flatrod), 6-10 (JH)   No date (CL),

 Added 2-7 .  Spotted it today on eBay

So now its up to 36 intakes.

Added 2-18  Spotted it today on an "E" code convertable

So now its up to 35 intakes.Also spotted a 3 29 at Carlise today.Its was already on the current list.

Oldmics