This is a focus ring attachment for a Voigtlander Nokton 50mm 1.1.
I found that after using the 28mm Nokton 2.0, the 50mm 1.1 was tricky to get used to being able to focus as quickly. I suppose over time that this could get easier as I got more accustomed to the lens but decided to do something about it in the mean time.
The tab is vaguely modeled from the 28mm focus tab so should be a very similar size when printed and in use.
The ring should slip onto the lens from the rear as it will not fit over the aperture ring due to fine tolerances. The fit will be very slightly snug and seems to be enough to hold itself in place. That being said, it is still loose enough that it could get knocked off the lens if you aren't careful. I didn't want to put stresses on the lens barrel.
I'm not inclined to use any adhesive but perhaps some blu-tak (or equivalent) under one or two of the scalloped sections of the ring would be enough to ensure a more solid feel.