User blog comment:ZokeiR/Possible Grand Servants/@comment-37711538-20200107235921

This is an old thread, but I'm gonna add my two cents anyway.

I read somewhere on the Type-Moon Wiki that Proto Arthur was currently on the search for a Beast-Class Servant (Beast 666(?), and seeing as how Grand Servants are summoned whenever a Beast is near -- from what I've seen/read/heard --, I'm pretty sure that automatically means he's got the title of Grand Saber in the bag.

If not, then it goes to Artoria as you said.