Commentaire sur Les Nombres 26:24

לישוב OF JASHUB — He must be identical with Job (Genesis 46:13) who is mentioned amongst those who went down to Egypt, for all the families were called by the names of those who went down to Egypt. But as for those who were born from that time and onwards their families were not called by their own names, except the families of Ephraim and Manasseh, both of whom wore born in Egypt, and Ard and Naaman, the sons of Bela the son of Benjamin (v. 40). And I have found in the work of R. Moses the Preacher why this was so in the case of the two latter — that their mother went down to Egypt when she was already pregnant with them so that they may be regarded as being among those who went down to Egypt, and on this account they formed separate families, just as Chamul and Chezron who were grandchildren of Judah (v. 21), and Cheber and Malkiel who were Asher’s grandchildren (v. 45). If this is an Agada, well and good; but if not, then I say that Bela had many grandchildren and that from these two, Ard and Naaman, there issued from each a large family, and the offspring of the other sons were called after Bela’s name, but the offspring of these two were called after their name. So, too, I say about the sons of Machir who formed two different families, one called after his name, and one called after the name of Gilead, his son, because it was a very large family. Five families are missing from Benjamin’s sons as stated before: here (i.e., by the fact that five of the families had become extinct) there was fulfilled part of his mother’s prophecy which is alluded to in the fact that she called him Ben Oni (my unfortunate son), (Genesis 35:18), whilst by the incident of the concubine in Gibea (Judges 20:35), the whole of it was fulfilled for practically the entire tribe was exterminated. This I found in the work of R. Moses the Preacher.

This is Yov… In Parshas Vayigash (Bereishis 46:13). Rashi explains in Divrei Hayomim I (7:1) that he was [really] called Yov, but since they settled down to learn Torah, as it is written (ibid. v.12), “From the sons of Yissachar there were those who had understanding of the times” he merited to be called “Yashuv” (lit. "settled").

לישוב, he is identical with Yov (in Genesis 46,13) in that one ש was added to him from the two ש of his father (יששכר), and therefore, (the 2nd Shin) is not read in his father's name.