https://en.wikipedia.org/wiki/Sarvastiv ... %C4%81dins"Willemen, Dessein, and Cox hold that this group is really the Sautrāntika school who renamed themselves in the later years of the Sarvāstivāda school history."
In their book "Sarvastivada Buddhist Scholasticism" they don't say that outrightly, there is only a few hints to that.
Also, where Warder say that
i don't remember anything like that in "Indian Buddhism". Is it in some paper?"Warder suggests that the Mūlasarvāstivādins was a late group who compiled a Vinaya and the Saddharmasmṛtyupasthāna Sūtra."