From what I've read, many do indeed contain real alprazolam, but it is also common to find etizolam, an Asian pharma benzo* sold as an RC in the West, which has similar effects to alprazolam (indeed etizolam is now getting federally scheduled because it's been found in so many presses, so it might become rarer in the future).
Other popular RC benzos that are turning up in these presses include clonazolam, diclazepam and flualprazolam (which is also about be scheduled).
*technically etizolam is a thienodiazepine not a benzodiazepine, but their structures and effects are so similar that they are generally referred to under the umbrella term "benzos".