Don't know about the casting numbers, but the earlier Chevrolet updraft Carter BB carbs had a center to center spacing of 2 11/16. Most common of these would be the 517s. Later carbs would have a center to center spacing of 2 15/16. Most common of these would be the 871s.

Jon.

Good carburetion is fuelish hot air
The most expensive carburetor is the wrong one you attempt to modify.
If you truly believe "one size fits all," try walking a mile in your spouse's shoes!
The Carburetor Shop