What is 1T? Given the context I am assuming tablespoon, but that’s not an abbreviation I’ve ever encountered before (tbsp being the only abbreviation I have seen).
Big T is tablespoon, little t is teaspoon. Probably not common since a tablespoon is 3x the size of a teaspoon and people would mix them up and the flavor profile would be wrong. You might encounter these abbreviations in cookbooks or from more experienced cooks for the sake of brevity.