Great question — answering as the UphillTech team:
What you're looking for is what's currently known as type C firecode. This board has improved burn rating to what's probably there (type X) info: http://www.awci.org/cd/pdfs/8309_f.pdf You can purchase type C in 1/2" or 5/8".
Full answer by Chris: https://diy.stackexchange.com/a/122943 (CC BY-SA 4.0).
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