bubble
Can't see it in the picture, but wonder if the bubble was there since new.
Not a lens expert or glass engineer, but from what I've read, a bubble can be a sign that it's made of the highest quality of glass, which is a good thing. To paraphrase what I've read (don't have the reference handy) the finest glass had to be melted at extreme high temperatures, and until recently the crucibles were not able to withstand the lengthy duration at these high temperatures, held to let the gases settle, and cooling down faster could trap gas. They (the crucibles with the molten glass inside) could reach the high temps, but not stay there long enough for the gases to rise up and escape, in some cases, before the temps were lowered for the cool-down process, and that this was due to the state-of-the-art of crucible material at one time, which has since been improved and eliminated this phenomenon. Poorer quality of glass was made at lower temps, held there to settle gas without problem, and did not challenge the limits of the crucible material. Trapped gas could be a sign of the finest quality of glass used in the production of the lens, and a side effect of quality material, not necessarily a defect, and is not detrimental to the lens from an optical standpoint.
This is from the reading I've done. Of course, I could be totally wrong.