I believe you are correct on that EC stamp. EC was, as said, an early smokeless powder was was one of the bulk powders. A 3-dram black powder "Measure" of it was to weigh 42-grains, so it along with Schultz was referred to as a 42-grain bulk powder. A later powder by EC I believe, but not sure the exact designation of it was developed which only weighed 36-grains for a 3-dram measure.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra