BOO and placing runner(s)

[dtwsd]

I understand this part of BOO but I do have a question:

After the incorrect batter has completed a turn at bat and:
a. BEFORE a legal or illegal pitch to the following batter
b. or before the pitcher and all infielders have vacated their normal fielding positions and have left fair territory:
EFFECT:
a. The player who should have batted is out.
b. Any advance of runners and any run scored shall be nullified. All outs made stand.

Here is my question regarding EFFECT (b) above.
Are runners returned to the base they had when the ball was put into play by the improper batter or are they returned to the base they had at the beginning of the improper
at-bat? Example:

R1 is at 1B with no outs when improper batter B3 bats for B2. On the first pitch R1 steals 2B. When the count reaches 2-2 improper batter B3 grounds out 4-3.
R1 advances to 3B on the play. Before the next pitch the defense appeals BOO. B3 remains out on the play. B2 is out for failing to bat, but where do we place R1?
Does she go back to 2B (where she was when the ball was put in play) or 1B (where she was at the beginning of the improper at-bat)?