http://www.tpub.com/gunners/6.htm
From the link:
M-6 and M-6+2.-M-6 and M-6+2 propellant designations equate to the SPDN and SPDF (respectively) descriptions previously provided. These designations are used to describe the propellants used in 76-mm ammunition. The +2 refers to a 2% mixture of potassium sulfate.
SPDN.- SPDN is a diphenylamine-stabilized smokeless powder to which nonvolatile materials are added to reduce the’ hydroscopic tendencies of the propellant. The N stands for nonhygroscopic.
SPD.- SPD is a single-base smokeless powder stabilized with diphenylamine.
Another useful link:
http://www.saami.org/specifications_and_information/publications/download/SAAMI_ITEM_200-Smokeless_Powder.pdf
76mm ammunition!? For what? An M4 Sherman tank?
Surely this stuff hasn’t been there since WWII.