Randy - I have compared the two side-by-side, and can see no obvious differences.
The following is undocumented hearsay:
Pontiac issued a block of numbers for the SD motors, so there is a different number for the SD. Even though the SD casting has a lower casting number, they were cast after production started with the standard 1960 3x2 manifold.
End of hearsay.
I have restored a number of what I believe to be original 1960 setups. The production 1960 tripowers that I have seen all had steel fuel lines, while the SD units that I have seen had aluminum. This tends to lend credence to the hearsay, as Pontiac went to the aluminum lines (mostly) for the 1961 production year.
Incidentally, there seems to be about as many of the SD manifolds as the standard manifold.
Jon.
__________________
"Good carburetion is fuelish hot air". 
"The most expensive carburetor is the wrong one given to you by your neighbor".
If you truly believe that "one size fits all" try walking a mile in your spouse's shoes!
Owner of The Carburetor Shop, LLC (of Missouri).
Current caretaker of the remains of Stromberg Caburetor, and custodian of the existing Carter and Kingston carburetor drawings.
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