Bomsaway, congratulations on your purchase...My second favorite car next to a 41.
I think the B refers to black top. The Fisher plant code for Baltimore is M. Incidentally all cabriolet bodies, painted and trimmed were built at the Fisher body plant in Lansing MI. then shipped to the various Chevrolet plants for final assembly.
Too bad the Chevrolet Motor Division tag is missing it contained the car number (serial number), assembly plant code and the month of the build. With the tag gone that information is gone too. you could "guesstimate" an approximate build month using the Fisher Body tag on the firewall, it will have the style number:40 1067 (1940 Cabriolet), body number: L (for Lansing) followed by a number representing the number of that body style down the line at that plant, starting with number 1. So with 11,820 cabriolets built, all of them at that plant, we get an average of 985 per month. See where your body number would have fit in... higher number, later build.
Also on that tag is the trim number (color of upholstery)and paint code. My 41 tag is hand stamped B T in the lower left corner for black and tan top.
Good luck with your project and come back for advice anytime.
Jay