Blank sheet enclosed inside sealed envelope - has visible writing produced by typewriter pressure (PT 21.6.78)

摘要

A sealed envelope encloses a blank sheet of paper (12). The front panel of the envelope is also provided with a transparent window (3). The two narrow sides (7) on the envelope edge are perforated (8) and they form guides when the continuous strip of envelopes is folded to a zig-zag stack. When a typewriter without ink ribbon is used to write a message onto the enclosed sheet (12) it produces visible print on the enclosed sheet due to the type pressure, without leaving any visible marks on the outside of the envelope. The address typed within the window space is visible due to its transparency.