I have checked around some military acronym sources and cannot find the acronym "d." anywhere. Discharged was designated "D", but that is not a cause of death.

There is some uncertainty around the acronym for ditto (do, D. and "), so it could be a typist literally typing their instructions. For example, if the soldier above your grandfather died from the same cause, the typist may have entered the ditto designation instead of the actual cause. It could have even been an actual shorthand symbol resembling a lowercase d.

I would suggest it might be worthwhile getting a full copy of your grandfather's service record. It will give you an idea of what kind of a man he was more so than just the death certificate.

For example, my grandfather ran away from a Catholic orphanage to join up at 16, went awol after a few weeks of basic training and was found working on the docks in another city, married at 17. He was given a reduced punishment and ended up serving as a refueller, not ever having been shot at.

Hello there! As your question is related to looking for identification/information regarding military personnel, our Guide on Military Identification may be of use to you. It provides a number of different resources, including how to request service records from a number of national agencies around the world, as well as graphical aids to assist in deciphering rank, unit, and other forms of badges or insignia. While the users here may still be able to lend you more assistance, hopefully this will provide a good place to start!