Hi Paul,
I'm guessing that you've got a 'D' code 'Bird with a 312... 'C' 'birds had 292s with a two barrel and aren't that common...
Original equipment per the '49-'59 Ford Text Catalog shows air horn I.D. numbers of:
ECZ-V or ECZ-Y [Fordomatic]; ECZ-AC or ECZ-AD [Fordomatic]; and B7AE 9510-A [no trans listed]
There's got to be something different about the three sets of numbers I've listed because the parts list breakdowns are in three separate listings.
That said, for your elevation of ~660 feet, all three carburetor listings show:
no. 55 primary jets and no. 48 secondary jets for 0 to 5,000 feet
Two 'economizers' are shown - (1) stamped No. 85, (1) stamped No. 105 (secondary)
'57 'D' 'Birds used the 4150 Holley which had metering blocks on both the primary and the secondary sides. While the parts book does not list the 85 as a 'primary' economizer valve, but does list the 105 as the economizer valve 'secondary', I believe each metering block has an 'economizer' valve. -Unfortunately, I could not verify this in the '49-'59 Ford Illustration Catalog parts blow-up picture.
Hope this helps.
NoShortcuts
a.k.a. Charlie Brown
near Syracuse, New York