Peephole.

Let me make ONE think clear. The backplate for a pushbutton doorbell is the PERFECT size and depth for a peephole plate.

 

PERFECT. I could almost cry.

 

Now, if any of my mid-century obsessed friends can assist me with finding a door knocker that is COOL and not just some awful looking knock-off I would be ENDLESSLY indebted to you :)

Done