In The Etymologies, the word for “craftsman, smith” was ᴹQ. tano, N. *tân as in N. Cirdan “Shipbuilder”, both from the root ᴹ√TAN “make, fashion”. In later writings, Tolkien changed √TAN >> √TAM “construct” and the Quenya form tano >> tamo “smith” (PE17/107-108), possibly due to conflict with a new root √TAN “show, indicate”. That this change was lasting is supported by the appearance of the form Q. Martamo as a title of Aulë in a “very late note” (LT1A/Talka Marda), as opposed to the form ᴹQ. Martan(o) in The Etymologies (Ety/TAN).
However, √TAM would have produced S. tav, and Tolkien wanted to retain the name Círdan. On PE17/108, he indicated that √TAN became the usual Sindarin form of the root, owing to the influence of √PAN “arrange, set in order”. Thus, the Sindarin development would have been √TAM > ✶tan- > tân, lenited in S. Círdan and S. calardan “lampwright”, whose plural appears in The Lord of the Rings as part of S. Rath Celerdain* “Lampwrights’ Street”.