The Bing carburetors used on the R51/3 to R69S models came with 3 different flanges.
We sometimes get the question: can I put carburetor X on BMW model Y?
In theory you can put an R51/3 carburetor on a R69 engine and even get the engine running, but you will quickly discover that the air pipes won’t fit.
These images are self explanatory
Modified carburetors can be found in the market that fit the R68, R69 and R69S cylinder heads.
However, there are some differences from the original models, most notably the carburetor flange:
