S. Parker Hardware 1601 Polished Brass US3 Finish Keystone Door Viewer Without Knocker. The 1601 keystone door interviewer has a 190 degree angle of view, and fits doors from 1/8" to 2 1/8" thick. There are frames for a number or name on the polished brass finish item. Used To Allow A User To Identify Visitors From The Security Of A Locked Door.