1/4" - 20 x 1" threaded stem.
Don't just guess or eye ball the thread size. Yours maybe different or metric.
If you are not sure, please check the thread size at your local hardware store. Don't just buy it and claim it doesn't fit.
Making sure will save you time, money and hassle.
If you want two casters with brake https://www.amazon.com/dp/B07B5PMGZ3
Go to your local hardware store to verify the thread size before ordering!!