Where is the data sourced for this checker?

I just tested two upcoming flights (UA2021 and UA2022), both of which are currently (as in, today) flown on 737-900. UA2021 shows as "Not a Max" and UA2022 shows as "Is a Max".

Yes, I fully realize Unided will swap these around as needed. Just curious where one would even find the "scheduled" airframe?