ציטירנדיקער פּירוש אויף שמות 1:3

ואלה שמות, seeing that the Torah wanted to let us know how the Israelites had increased and multiplied (verse 3) it became necessary to repeat that when they had arrived in Egypt they had numbered only 70 souls. The dramatic increase in numbers of Israelites began only with the death of the generation that had moved there from the land of Canaan. As a result, when a new king came to the throne in Egypt, he wanted to diminish their numbers and did not succeed in doing so.

It is remarkable that the stone with the name בנימין which was spelled plene, i.e. with two letters י (as opposed to Exodus 1,3 or Genesis 42,38 et al) where there is no letter י before the final letter ן filled out its stone seeing his name had six letters. It was therefore impossible to add a letter from the patriarchs’ names on his stone. The reason that this was so may be connected to a historical occurrence in the time of the Judges when the tribe came close to extinction, only 600 young men surviving after the civil war described in Judges 20-21. Had his name not been spelled with six letters on the breastplate, even the six hundred young men who survived might not have done so.