Is Humphrey a girl or boy?

(s)he was my Gramma's turtle that she took from the wild many, many years ago. Gma died in 2018 and I took Humphrey because she was severely neglected. I'm still not sure on Humphrey's sex, but I think she's female. She's a 50 year old Three-toed box turtle. Flat plastron and red eyes. See pics for details.