Jacqueline Kennedy is supposed to be on tape as saying she thought it was LBJ.
BTW, does everyone know the story (and correct me if I am wrong), that the "bible" LBJ is swearing his oath on in the famous airplane picture, with Jackie next to him, is actually a Latin Mass missal?
He wanted to wait until this particular (woman) judge could come and swear him in, and meanwhile everyone was hunting around for a bible for him to use.
At the time, Air Force One was still full of Kennedy staff, and all that could be found was a missal someone had. So, they used it.