Ground rule double?

[Paul L]

Home run fence stops at the foul lines, leaving a fifteen foot gap between it and the fences marking dead ball territory. B1 hits a hard grounder fair past third. It rolls over the left-field line into foul ball territory, then through the aforementioned gap, and keeps rolling. B1 touches 'em all while defensive coach is yelling for a ground rule double. No such ground rule was discussed in the pregame conference.

Fed Rule 5-1-1-f says that a fair batted ball over fair ground is dead if it goes over, through, or wedges in a fence. 8-4-3-i says runner advances without liability when fair ball bounces over or rolls under or through a fence or any designated boundary of the playing field.

Where should B1 be after the play? Why?