Toronto. Leitz may have just as well used a variant of the early iPhone tag line: “There’s an app for that”, using, “There’s an acc (accessory) for that” instead. In the early years, Leitz made slip-on filters for the petite 5cm Elmar. But they used a front tab to adjust the aperture.
If you wanted to change the aperture, you had to remove the lens hood, remove the filter, adjust the lens aperture, replace the filter, replace the lens hood, then snap the subject – if it was still around.
To solve this issue and speed things up, Leitz made and sold a thin flexible ring called a VOOLA , later coded as 16621. The ring fit the filter (or lens hood) allowing the snap-shooter to adjust the aperture quicker. A dirt cheap solution to a troublesome problem. Remember, in the pre war days there was no lens coating so a hood was essential to protect the already poor lens contrast from indirect sunshine.
In 1984, Hove Foto Books in the UK published a hard cover pocket book called “Leica Accessory Guide. This was just one of its many historic Leica books. Page 105 (top part shown here) included the VOOLA under “Filters, Hoods, Adaptors, etc.”. Ringy Thingy anyone?