Harry;
The 27.gBl ? is 27 grams of lead. That would be the shot load which is just shy of an ounce, about 15/16oz. I have not looked at a lot of German guns & had not run into the N/A before, but I believe this is not the proof charge, but the service charge. Using Schultz powder this would be for the shot bbls. Shultz was a bulk smokeless powder which was bulked up so that a "Dram Equivelent" charge of it could be measured in a black powder measure. A 3 dram "Measure" of Schultz was set as weing 42 grains. A 2.9 gram measure would weigh about 44.75 grains or about 3 3/16 dram equivelent load.
I forget the exact spelling now but Bl is an abbreviation of a German word meaning lead. Geschoss(Spelling) is a German word for essentially bullet. One often sees the mark BlG on rifle bbls proofed for use with lead bullets. This "May" be what your BG is with the l omitted in the stamping, not sure.
