In Rom 2:27,29means the outward form. The "oldness of the letter" (7:6) is a phrase which denotes the old way of literal outward obedience to the law as a system of mere external rules of conduct. In 2 Corinthians 3:6, "the letter" means the Mosaic law as a written law. (See WRITING .)
These dictionary topics are from M.G. Easton M.A., D.D., Illustrated Bible Dictionary, Third Edition, published by Thomas Nelson, 1897. Public Domain, copy freely.
Easton, Matthew George. "Entry for 'Letter'". "Easton's Bible Dictionary".