Looking at the parts catologue the difference seems to be which way the bolt holes are situated for the carb, 480.378 has them front to rear on the manifold and 481.718 for the later 32 and 34 PBIC carbs are right to left across the manifold
If it’s 480.378 Fransen has one for 25.76 euro
https://www.citroen-traction-avant.com/en/shop/article/480.378/Heat_conduction_central_bore_38_mm
David Faulkner