I have a 1907 drilling 16-16-8x58R which has that same load marking on the sides of the shot bbls except accompanied by the early nitro mark ie a crown over N. I too have always taken this as 2 grams of Schultz powder. Schultz was a bulk smokeless powder which originated in Germany but became highly popular in England as well & also the US. It was classified as a 42 grain powder that is a 3 dram measure of it weighed 42 grains. This 2 gram load then wouls correspond to a 2.2 dram measure, a little lighter than normal for a 1oz 16ga load, which as Steve said was normally 2½ drams. I can't really verify it but have read in the past that German law prior to 1912 made no distinction in proof charges for black & smokeless, ie percentage increase, not actual wts. This it was said resulted in a gun given a smokeless proof being submitted to a very excessive charge. Most makers didn't submit their guns for the "supplemental" smokeless proof, but if one was sumitted at a customer's request a lighter than normal load was specified to keep the proof charge to a lower level.
In 1912 the rules were changed regarding the charge & smokeless proof became mandatory as I recall.
