Toronto. When Leitz marketed their famous little Leica in 1925, it had a squinty little 5cm viewfinder that remained unchanged for the life of the screw mount cameras. Worse, by 1930, Leitz had added lenses of other focal lengths and made the Leica interchangeable.
To compensate for this, the company offered a slew of better viewfinders which could be slipped into the accessory shoe. Most could be adjusted for vertical parallax correction. These gave the photographer a clearer view of his subjects and framed the captured area for lenses besides the traditional 5cm lenses (Elmar, Summar, Summicron, etc.).
The viewers came in sports frame versions like the RASAL example shown above. Many had means to adjust to various focal lengths like the VIDOM, VIOOH, or the Leitz NY IMFIN, usually known as an Imarect. Many viewers offered bright lines marking the boundary of the focal length while allowing the photographer to see subjects about to enter the field of view. Some where collapsible to make them more compact to carry.
While helping the photographer to see better, these little accessories made a profitable after market for Leitz. You could buy the externally mounted viewers in many different varieties for the same lens to suit your specific needs.
N.B. During the Cuban Missile crisis the fall of 1962 a song called “Do You Hear What I Hear?” was published. The above title was a line in that song. The song is now a popular Christmas song heard every December.