The original bead was possibly within the size of the dotted line.
So what...?!

The lower part of the bead had most likely broken off or was otherwise damaged. The owner of the bead felt it had to be repaired - so she did. The repair - with glass!!! - is an amazing job, not just under Saharan conditions. Actually a masterful repair (leaving the application of the new design unconsidered, as TASART pointed out).

What I learnt from this is possibly something about the approximate age of the bead. And certainly something of the owners appreciation of his obviously beloved piece.

Why something can be learnt about the age?

My hypothesis is not fully conclusive, since other explanations are possible too. But I like this version:

Somebody who goes through the process of repairing such a bead (instead of finding an alternative) must truely LOVE the piece. Such a woman is not likely to sell her beloved bead, after repair. Most likely she kept for quite a while, after the succesful reconstruction.

Still - the bead was sold at some point. To me, actually!

We can now suspect the bead remained in the possession of this owner until her death - hence my believe this bead might have a certain age!

Another argument speaks for an older age of the bead. The actual generation of beadmakers (since ca. 1980-85) in Mauritania would NEVER (EVER) been able to execute such a masterful repair, leading to the assumption the bead is definitely and AT LEAST 30 years old. Realistic is an age of 50-60 years+!