Of course, we can't conclusively rule out "some medal of a chamber of industry and commerce, or something similar," but do you have any examples? There are, indeed, precedents for ribbons of Bavarian medals in this non-military direction to be something other than the usual Bayern blau/weiss - for example the 1910 Agricultural Jubilee Medal...
Obviously, this isn't the medal on the bar - I provide it only as an example that Bavaria used other color ribbons - including green/red. But I can't find any Bavarian medal with the subject ribbon. Given the white, green red of our subject ribbon - the usual suspects would be Bulgarian, Hungary, or Italy. Of those, the closest is the Bulgarian Civil Merit and it seems to make sense on this bar - considering the lack of a MVK and the presence of the fireman's medal. In my neophyte opinion, of course. Nonetheless, there is still room for doubt ad you are correct - the word "must" is too strong. But the Bulgarian Civil Merit is the "most" likely suspect presented so far...