My two cents.
On the M-88, not one of the military factory photos I have (and have many) show a bare factory installed carb. All were either nickel, silver, green, or black.
Military contracts for the WLA specify that there will be no bare metal parts. The military was concerned with corrosion. This being a bronze carburetor body would be subject to corrosion.
On the personal side, I have acquired many NOS M-series carbs over the last 35 years and all had a finish on the body. The one above is the first I have seen without paint. If delivered without paint would have been in violation of the military contracts (not that it could never happen). Leads me to believe someone has removed the original finish sometime in the last 60 years. Particularly since the patina of the throttle lever comes no where to the patina of the body. (During the mid-50's, all parts stocks -tanks, trucks, motorcycles, etc - were repackaged and repainted if necessary. This may have been stripped at that time for a repaint dur to corrosion, as evidenced by the corroded cad palting, but then overlooked. This program was handled Ordnance personnel and some outside contractors, not Linkert not H-D.)
Regardless of the origin of the finish of this particular replacement M-88, the original question dealt with the finish on a Linkert M-series carburetor as it left the factory installed on the motorcycle. I will re-check all post-war photos of civilian models (none K) to see if I can find anything other than a black carb and let you know. Up front, I think I am going to be re-verifing that they are all black. I doubt if the factory was hap hazard about the appearance of new motorcycles and would have insured uniformity. And yes, non-standard parts show up on assembly lines, but the good lineman would rather kick the part back then have to later remove and replace it.