In Wirnsberger's Standard directory of proof marks on page 123 he lists several powders said to be suitable for use in semi-smokeless proof which could be specified by the applicant. Among these under British powders is listed EC3 with the 3 being writen as an exponent. I think Larry has nailed this mark as being a specific lot/type of EC powder, even though the M is not shown in this reference.


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