htg = "Honest-To-God"
The use of the term term "HTG" comes from the days when they were developing the atomic bomb in the Manhattan Project. They needed a code word for plutonimum, so they called it "calcium" in all the documents and correspondence.
Well, by and by, the scientists found that real metallic calcium had to be used for something in their top-secret research. They were in a bit of a quandary as to what to call the real calcium... they couldn't use the term "real calcium," since that would give away the fact that "calcium" actually was a code word for something else --that is, plutonium.
So they settled on the term "HTG calcium" for the real calcium.
"So we'll have to buy some HTG Calcium for the rest of the experiments with calcium."