Cast Brass body. Brass shackle. Pin tumbler. Flat key. We did not find a patent for this lock. Jon Flatt wrote: This lock was not made for the Post Office. I worked at a small grocery store when I was in high school. I helped unload the tractor trailer when it came in with groceries. I saw him writing down something just before he opened the back door of the trailer and I asked him what he was doing and he asked me to verify the numbers and initial a book he had. It was this type of lock.”James R. Mundy collection at Railway Mail Service Library, gift of Bob DeWittie” This padlock requires further research