E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Pen and Feather
are varieties of the same word, the root being the Sanskrit pat, to fly. (We have the Sanskrit pattra, a wing or instrument for flying; Latin, petna on penna, pen; Greek, pteron; Teutonic, phathra; Anglo-Saxon, fether; our feather.)
Analogous examples are TEAR and LARME, NAG and EQUUS, WIG and PERUKE, HEART and CUR, etc.