Maay usually means "where", but can also mean "which one?" (usually of two choices, per Mesa Grande dictionary).
This form has -vech, with -ch marking it as the subject, as in, "Which one did it? Which one was it?"
Maayvech shaawiivu shuuhuua? "Which one stole the shawii?"
Maayvech 'equll 'ehanchu? "Which one is longer?"